v3.25.4
Derivative Financial Instruments and Hedge Accounting - Summary of Outstanding Derivative Financial Instruments (Detail) - GBP (£)
£ in Millions
Dec. 31, 2025
Dec. 31, 2024
Disclosure of detailed information about hedges [line items]    
Gross notional amounts £ 1,531 £ 3,130
Assets 16 51
Liabilities (3) (58)
In less than one year [member]    
Disclosure of detailed information about hedges [line items]    
Gross notional amounts 1,156 2,505
Assets 2 31
Liabilities (1) (54)
One to five years [member]    
Disclosure of detailed information about hedges [line items]    
Gross notional amounts 75 325
Assets 0 3
Liabilities (1) (1)
Greater than five years [Member]    
Disclosure of detailed information about hedges [line items]    
Gross notional amounts 300 300
Assets 14 17
Liabilities (1) (3)
Interest rate derivatives [member] | Fair value hedges [member]    
Disclosure of detailed information about hedges [line items]    
Gross notional amounts 0 166
Assets 0 0
Liabilities 0 (1)
Interest rate derivatives [member] | Not designated in hedging relationship [member]    
Disclosure of detailed information about hedges [line items]    
Gross notional amounts 375 779
Assets 14 22
Liabilities (2) (6)
Cross currency rate derivatives [member] | Fair value hedges [member]    
Disclosure of detailed information about hedges [line items]    
Gross notional amounts 0 342
Assets 0 21
Liabilities 0 (32)
Cross currency rate derivatives [member] | Not designated in hedging relationship [member]    
Disclosure of detailed information about hedges [line items]    
Gross notional amounts 0 83
Assets 0 0
Liabilities 0 (4)
FX derivatives [member] | Fair value hedges [member]    
Disclosure of detailed information about hedges [line items]    
Gross notional amounts 993 1,049
Assets 2 0
Liabilities (1) (15)
FX derivatives [member] | Not designated in hedging relationship [member]    
Disclosure of detailed information about hedges [line items]    
Gross notional amounts 163 711
Assets 0 8
Liabilities £ 0 £ 0