Subclassifications Of Assets, Liabilities And Equities [Abstract]
| Concept | As at 2024-12-31 | As at 2023-12-31 | 
|---|---|---|
| Subclassifications of assets, liabilities and equities [abstract] | ||
| Property, plant and equipment [abstract] | ||
| Property, plant and equipment | 
                                                48,600,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                56,400,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Intangible assets and goodwill [abstract] | ||
| Intangible assets and goodwill | 
                                                9 9. 7
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                152,600,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Trade and other non-current receivables [abstract] | ||
| Trade and other non-current receivables | 
                                                4,500,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                5,200,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Miscellaneous non-current assets [abstract] | ||
| Non-current net defined benefit asset | 
                                                4. 1
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                4,200,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Non-current derivative financial assets | 
                                                –
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                2 .3
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Trade and other current receivables [abstract] | ||
| Trade and other current receivables | 
                                                38,700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                47 .3
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Current prepayments and current accrued income including current contract assets [abstract] | ||
| Current accrued income including current contract assets [abstract] | ||
| Current contract assets | 
                                                500,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                1,800,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Classes of current inventories [abstract] | ||
| Current inventories | 
                                                82,500,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                94,500,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Classes of current inventories, alternative [abstract] | ||
| Cash and cash equivalents [abstract] | ||
| Cash and cash equivalents | 
                                                57 .3
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                8. 7
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Non-current assets or disposal groups classified as held for sale or as held for distribution to owners [abstract] | ||
| Non-current assets or disposal groups classified as held for sale or as held for distribution to owners | 
                                                –
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                12 .3
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Miscellaneous current assets [abstract] | ||
| Current derivative financial assets | 
                                                800,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                1,800,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Classes of other provisions [abstract] | ||
| Other provisions [abstract] | ||
| Other non-current provisions | 
                                                700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                800,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Borrowings [abstract] | ||
| Non-current portion of non-current borrowings | 
                                                114,200,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                9 9. 0
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Current borrowings and current portion of non-current borrowings [abstract] | ||
| Current borrowings and current portion of non-current borrowings | 
                                                200,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                200,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Non-current portion of non-current borrowings, by type [abstract] | ||
| Current borrowings and current portion of non-current borrowings, by type [abstract] | ||
| Trade and other non-current payables [abstract] | ||
| Trade and other non-current payables | 
                                                800,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                1,200,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Trade and other current payables [abstract] | ||
| Current accruals and current deferred income including current contract liabilities [abstract] | ||
| Current deferred income including current contract liabilities [abstract] | ||
| Current contract liabilities [abstract] | ||
| Current contract liabilities | 
                                                4,200,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                2 .1
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Trade and other current payables | 
                                                43,700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                42,800,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Miscellaneous non-current liabilities [abstract] | ||
| Non-current net defined benefit liability | 
                                                2,500,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                2 .9
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Miscellaneous current liabilities [abstract] | ||
| Current derivative financial liabilities | 
                                                300,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                0 .1
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Issued capital [abstract] | ||
| Issued capital | 
                                                18. 9
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                18. 9
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Retained earnings [abstract] | ||
| Retained earnings | 
                                                52,200,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                93,400,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Accumulated other comprehensive income [abstract] | ||
| Reserve of cash flow hedges | 
                                                400,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                2 .9
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Miscellaneous equity [abstract] | ||
| Capital redemption reserve | 
                                                1,600,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                1,600,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Net assets (liabilities) [abstract] | ||
| Assets | 
                                                346,300,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                451,300,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Liabilities | 
                                                260,400,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                213,800,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Assets (liabilities) | 
                                                85. 9
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                237 .5
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Net current assets (liabilities) [abstract] | ||
| Current assets | 
                                                188,700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                159 .8
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Current liabilities | 
                                                118,800,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                65,700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Assets less current liabilities [abstract] |