v3.25.4
Retirement Benefit and Other Post-retirement Obligations - Summary of Changes in Value of US PRMB (Detail) - GBP (£)
£ in Millions
12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Disclosure of defined benefit plans [line items]      
Opening defined benefit obligation £ (19)    
Interest on plan liabilities 136 £ 122 £ 127
Closing defined benefit obligation (17) (19)  
PRMB [member]      
Disclosure of defined benefit plans [line items]      
Interest on plan liabilities 1 1 0
Present value of defined benefit obligation [member]      
Disclosure of defined benefit plans [line items]      
Exchange differences 4 0  
Interest on plan liabilities (135) (121)  
Actuarial gains – experience (52) (53)  
Actuarial losses – financial 17 162  
Benefits paid 165 154  
Present value of defined benefit obligation [member] | PRMB [member]      
Disclosure of defined benefit plans [line items]      
Opening defined benefit obligation (19) (21)  
Exchange differences 2 0  
Interest on plan liabilities (1) (1)  
Actuarial gains – experience (1) 1  
Actuarial losses – financial 0 0  
Benefits paid 2 2  
Closing defined benefit obligation £ (17) £ (19) £ (21)