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Retirement Benefit and Other Post-retirement Obligations - Summary of Changes in Value of US PRMB (Detail) - GBP (£)
£ in Millions
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Disclosure of defined benefit plans [line items]      
Opening defined benefit obligation £ (39)    
Interest on plan liabilities 53 £ 63 £ 81
Closing defined benefit obligation (34) (39)  
PRMB [member]      
Disclosure of defined benefit plans [line items]      
Interest on plan liabilities 1 1 2
Present value of defined benefit obligation [member]      
Disclosure of defined benefit plans [line items]      
Exchange differences (1) 3  
Interest on plan liabilities (52) (62)  
Actuarial gains/(losses) - experience (97) (20)  
Actuarial gains/(losses) - demographic (1) 2  
Actuarial gains/(losses) - financial 166 (310)  
Benefits paid 133 133  
Present value of defined benefit obligation [member] | PRMB [member]      
Disclosure of defined benefit plans [line items]      
Opening defined benefit obligation (39) (43)  
Exchange differences (1) 1  
Interest on plan liabilities (1) (1)  
Actuarial gains/(losses) - experience 2 3  
Actuarial gains/(losses) - demographic 0 1  
Actuarial gains/(losses) - financial 2 (3)  
Benefits paid 3 3  
Closing defined benefit obligation £ (34) £ (39) £ (43)