Analysis Of Income And Expense [Abstract]
| Concept | 2023-01-01 to 2023-12-31 | 2023-01-01 to 2023-12-31 | 2023-01-01 to 2023-12-31 | 2023-01-01 to 2023-12-31 | 2023-01-01 to 2023-12-31 | 2022-01-01 to 2022-12-31 | 2022-01-01 to 2022-12-31 | 2022-01-01 to 2022-12-31 | 2022-01-01 to 2022-12-31 | 2022-01-01 to 2022-12-31 | 
|---|---|---|---|---|---|---|---|---|---|---|
| Analysis of income and expense [abstract] | ||||||||||
| Revenue [abstract] | ||||||||||
| Revenue | — | 
                                                491,000,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | — | — | 
                                                426,000,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | 
| Income tax relating to components of other comprehensive income [abstract] | ||||||||||
| Income tax relating to components of other comprehensive income that will not be reclassified to profit or loss | — | 
                                                100,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | — | — | — | 
                                                2,400,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | — | 
| Comprehensive income, continuing and discontinued operations [abstract] | ||||||||||
| Comprehensive income | 
                                                –
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                6,100,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                6,500,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                –
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                400,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                –
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                200,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                15,000,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                –
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
                                                15,200,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             |