Statement Of Cash Flows [Abstract]
| Concept | As at 2024-12-31 | 2024-01-01 to 2024-12-31 | As at 2023-12-31 | 2023-01-01 to 2023-12-31 | 
|---|---|---|---|---|
| Statement of cash flows [abstract] | ||||
| Cash flows from (used in) operating activities [abstract] | ||||
| Interest paid, classified as operating activities | — | 
                                                10,300,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                15,400,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Income taxes paid (refund), classified as operating activities | — | 
                                                500,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                10,500,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Cash flows from (used in) operating activities | — | 
                                                12,700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                16. 1
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Cash flows from (used in) investing activities [abstract] | ||||
| Cash flows used in obtaining control of subsidiaries or other businesses, classified as investing activities | — | 
                                                –
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                1,600,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Proceeds from sales of property, plant and equipment, classified as investing activities | — | 
                                                2,700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                200,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Purchase of property, plant and equipment, classified as investing activities | — | 
                                                7 . 9
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                4,800,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Proceeds from sales of other long-term assets, classified as investing activities | — | 
                                                –
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                0 .9
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Interest received, classified as investing activities | — | 
                                                200,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                –
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Cash flows from (used in) investing activities | — | 
                                                12,600,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                20 .8
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Cash flows from (used in) financing activities [abstract] | ||||
| Proceeds from issuing shares | — | 
                                                –
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                118. 1
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Payments to acquire or redeem entity's shares | — | 
                                                0 .5
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                3,700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Proceeds from borrowings, classified as financing activities | — | 
                                                244,700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                240,000,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Repayments of borrowings, classified as financing activities | — | 
                                                231. 1
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                313. 9
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Payments of lease liabilities, classified as financing activities | — | 
                                                6. 1
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                6,700,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Dividends paid, classified as financing activities | — | 
                                                –
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                11,600,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Cash flows from (used in) financing activities | — | 
                                                7. 0
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                23,400,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Increase (decrease) in cash and cash equivalents before effect of exchange rate changes | — | 
                                                7. 1
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                13,500,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Effect of exchange rate changes on cash and cash equivalents [abstract] | ||||
| Effect of exchange rate changes on cash and cash equivalents | — | 
                                                1,100,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                2,400,000
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | 
| Cash and cash equivalents | 
                                                57 .3
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — | 
                                                8. 7
                                                
                                                    
                                                        GBP
                                                    
                                                
                                             | — |