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Accounting Policies (Detail) - GBP (£)
£ in Millions
Jun. 30, 2022
Dec. 31, 2021
Jun. 30, 2021
 [1]
Dec. 31, 2020
Disclosure of changes in accounting estimates [line items]        
Equity £ 4,270.4 £ 4,069.0 £ 4,686.3 £ 5,050.1 [1]
Retained earnings        
Disclosure of changes in accounting estimates [line items]        
Equity 3,909.2 4,367.3 4,840.6 4,959.2 [1]
Other reserves        
Disclosure of changes in accounting estimates [line items]        
Equity 274.0 £ (335.9) £ (48.2) 191.2 [1]
Increase (decrease) due to corrections of prior period errors [member]        
Disclosure of changes in accounting estimates [line items]        
Equity       £ 116.3
Increase (decrease) due to corrections of prior period errors [member] | Retained earnings        
Disclosure of changes in accounting estimates [line items]        
Equity (111.5)      
Increase (decrease) due to corrections of prior period errors [member] | Other reserves        
Disclosure of changes in accounting estimates [line items]        
Equity £ (4.8)      
[1] Figures have been restated as described in Note 2: Accounting Policies.