Disclosure Of Entity's Operating Segments [Text Block]
| Concept | 2023-12-31 to 2024-12-28 | 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 | 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 | 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 | 
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Disclosure of entity's operating segments [text block] | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Disclosure of operating segments [text block] | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Disclosure of operating segments [abstract] | ||||||||||||||||
| Disclosure of operating segments [line items] | ||||||||||||||||
| Revenue | — | — | 
                                                2,014,400,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | 
                                                –
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                2, 014.4
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                –
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                1,809,600,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                1,809,600,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | — | — | 
| Tax expense (income) | — | — | 
                                                50,500,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | 
                                                1,700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                48,800,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                4,800,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                4 1.0
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                45,800,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | — | — | 
| Profit (loss) before tax | — | — | 
                                                203,900,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | 
                                                14,100,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                189,800,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                20,600,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                167 .7
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                188,300,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | — | — | 
| Profit (loss) | 
                                                –
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                –
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                153,400,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                –
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                153,400,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                12,400,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                14 1.0
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                15,800,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                126,700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                142,500,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                –
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                –
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                142,500,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                –
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Assets | — | — | — | 
                                                1,319,700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | 
                                                1,129,700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Liabilities | — | — | — | 
                                                749,200,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | — | — | 
                                                598,800,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
| Cash flows from (used in) operating activities | — | — | 
                                                310,900,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | — | — | — | — | 
                                                3 10.8
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | — | — | 
| Cash flows from (used in) investing activities | — | — | 
                                                217 .1
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | — | — | — | — | 
                                                191,200,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | — | — | 
| Cash flows from (used in) financing activities | — | — | 
                                                163,800,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | — | — | — | — | 
                                                115,900,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | — | — | 
| Disclosure of products and services [text block] | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Disclosure of products and services [abstract] | ||||||||||||||||
| Disclosure of products and services [line items] | ||||||||||||||||
| Disclosure of geographical areas [text block] | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Disclosure of geographical areas [abstract] | ||||||||||||||||
| Disclosure of geographical areas [line items] | ||||||||||||||||
| Disclosure of major customers [text block] | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | — | 
| Disclosure of major customers [abstract] | ||||||||||||||||
| Disclosure of major customers [line items] |