Analysis Of Income And Expense [Abstract]
| Concept | 2023-12-31 to 2024-12-28 | 2023-12-31 to 2024-12-28 | 2023-12-31 to 2024-12-28 | 2023-12-31 to 2024-12-28 | 2023-01-01 to 2023-12-30 | 2023-01-01 to 2023-12-30 | 2023-01-01 to 2023-12-30 | 2023-01-01 to 2023-12-30 | 
|---|---|---|---|---|---|---|---|---|
| Analysis of income and expense [abstract] | ||||||||
| Revenue [abstract] | ||||||||
| Interest income | — | — | 
                                                400,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | 
                                                700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Revenue | — | — | 
                                                96,000,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | 
                                                88,000,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Material income and expense [abstract] | ||||||||
| Finance income (cost) | — | — | 
                                                300,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | 
                                                500,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Operating expense | — | — | 
                                                23,200,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | 
                                                21,700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Comprehensive income, continuing and discontinued operations [abstract] | ||||||||
| Comprehensive income | 
                                                -
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                -
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                3,600,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                3,600,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                -
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                2,700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                2,700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                -
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             |