Statement Of Changes In Equity [Abstract]
| Concept | 2023-12-31 to 2024-12-28 | 2023-12-31 to 2024-12-28 | As at 2024-12-28 | 2023-12-31 to 2024-12-28 | As at 2024-12-28 | As at 2024-12-28 | 2023-12-31 to 2024-12-28 | As at 2024-12-28 | As at 2023-12-30 | As at 2023-12-30 | 2023-01-01 to 2023-12-30 | As at 2023-12-30 | 2023-01-01 to 2023-12-30 | 2023-01-01 to 2023-12-30 | As at 2023-12-30 | 2023-01-01 to 2023-12-30 | As at 2022-12-31 | As at 2022-12-31 | As at 2022-12-31 | As at 2022-12-31 | 
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Statement of changes in equity [abstract] | ||||||||||||||||||||
| Statement of changes in equity [line items] | ||||||||||||||||||||
| Equity | — | — | 
                                                9,900,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                37,600,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                300,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                27,400,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                7,800,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                27,400,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                300,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | 
                                                35,500,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                34,000,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                24,600,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                9,100,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                300,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Changes in equity [abstract] | ||||||||||||||||||||
| Comprehensive income [abstract] | ||||||||||||||||||||
| Profit (loss) | 
                                                -
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                -
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                3,600,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | 
                                                3,600,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | 
                                                -
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                2,700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                2,700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                -
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | — | 
| Other comprehensive income | — | — | — | 
                                                -
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | 
                                                -
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | — | — | — | 
| Comprehensive income | 
                                                -
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                -
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                3,600,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | 
                                                3,600,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | 
                                                -
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                2,700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                2,700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                -
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | — | 
| Issue of equity | — | — | — | — | — | — | — | — | — | — | 
                                                2,800,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                2,800,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                -
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                -
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | — | 
| Dividends recognised as distributions to owners | — | — | — | — | — | — | — | — | — | — | 
                                                -
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                1,400,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                1,400,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                -
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | — | 
| Increase (decrease) through share-based payment transactions, equity | 
                                                -
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                -
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                500,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | 
                                                500,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | 
                                                -
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                200,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                200,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                -
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | — |