Subclassifications Of Assets, Liabilities And Equities [Abstract]
| Concept | As at 2024-12-28 | As at 2023-12-30 | As at 2022-12-31 | 
|---|---|---|---|
| Subclassifications of assets, liabilities and equities [abstract] | |||
| Property, plant and equipment [abstract] | |||
| Property, plant and equipment | 
                                                664,700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                510,300,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Intangible assets and goodwill [abstract] | |||
| Intangible assets and goodwill | 
                                                24,900,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                18,300,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Investments in subsidiaries, joint ventures and associates reported in separate financial statements [abstract] | |||
| Investments in subsidiaries reported in separate financial statements | 
                                                –
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                –
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Miscellaneous non-current assets [abstract] | |||
| Non-current net defined benefit asset | 
                                                –
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                6,600,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Trade and other current receivables [abstract] | |||
| Trade and other current receivables | 
                                                62,400,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                53,800,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Classes of current inventories [abstract] | |||
| Current inventories | 
                                                55,200,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                48,800,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Classes of current inventories, alternative [abstract] | |||
| Cash and cash equivalents [abstract] | |||
| Cash and cash equivalents | 
                                                125,300,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                195,300,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                191,600,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Trade and other non-current payables [abstract] | |||
| Other non-current payables | 
                                                1,800,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                2,300,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Trade and other current payables [abstract] | |||
| Trade and other current payables | 
                                                243,900,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                211,100,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Miscellaneous current liabilities [abstract] | |||
| Current net defined benefit liability | 
                                                400,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                –
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Issued capital [abstract] | |||
| Issued capital | 
                                                2 .0
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                2,000,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Retained earnings [abstract] | |||
| Retained earnings | 
                                                543,000,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                503,400,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Miscellaneous equity [abstract] | |||
| Capital redemption reserve | 
                                                400,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                400,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Net assets (liabilities) [abstract] | |||
| Assets | 
                                                1,319,700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                1,129,700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Liabilities | 
                                                749,200,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                598,800,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Assets (liabilities) | 
                                                570,500,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                530,900,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Net current assets (liabilities) [abstract] | |||
| Current assets | 
                                                242,900,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                297 .9
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Current liabilities | 
                                                310,200,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                272,500,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Assets less current liabilities [abstract] |