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 | 
                                                900,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                1,100,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Intangible assets and goodwill [abstract] | |||
| Intangible assets other than goodwill [abstract] | |||
| Intangible assets other than goodwill | 
                                                12,000,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                11,600,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Goodwill | 
                                                13,700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                13,300,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Miscellaneous non-current assets [abstract] | |||
| Non-current investments other than investments accounted for using equity method | 
                                                100,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                1,100,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Trade and other current receivables [abstract] | |||
| Trade and other current receivables | 
                                                19,000,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                15,300,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Classes of current inventories [abstract] | |||
| Current inventories | 
                                                100,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                -
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Classes of current inventories, alternative [abstract] | |||
| Cash and cash equivalents [abstract] | |||
| Cash and cash equivalents | 
                                                10,900,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                10,700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                27,000,000
                                                
                                                    
                                                        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 | 
                                                -
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                1,000,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Trade and other current payables [abstract] | |||
| Trade and other current payables | 
                                                21,400,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                19,900,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Issued capital [abstract] | |||
| Issued capital | 
                                                300,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                300,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Retained earnings [abstract] | |||
| Retained earnings | 
                                                9,900,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                7,800,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Net assets (liabilities) [abstract] | |||
| Assets | 
                                                60,300,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                57,400,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Liabilities | 
                                                22,700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                21,900,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Assets (liabilities) | 
                                                37,600,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                35,500,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Net current assets (liabilities) [abstract] | |||
| Current assets | 
                                                30,800,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                26,000,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Current liabilities | 
                                                22,300,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                21,600,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Assets less current liabilities [abstract] |