v3.25.4
Deferred Income Tax - Schedule of Detailed Information About Unrecognised Tax Losses Expiry Dates Explanatory (Detail) - GBP (£)
£ in Millions
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Disclosure of Detailed Information About Unrecognised Tax Losses [line Items]    
Gross Amount £ 896 £ 965
Tax Effected 186 200
Within 10 years [member]    
Disclosure of Detailed Information About Unrecognised Tax Losses [line Items]    
Gross Amount 425 464
Tax Effected 89 97
Within 20 years [member]    
Disclosure of Detailed Information About Unrecognised Tax Losses [line Items]    
Gross Amount 117 135
Tax Effected 6 7
Available indefinitely [member]    
Disclosure of Detailed Information About Unrecognised Tax Losses [line Items]    
Gross Amount 354 366
Tax Effected 91 96
UK [member]    
Disclosure of Detailed Information About Unrecognised Tax Losses [line Items]    
Gross Amount 167 167
Tax Effected 42 42
UK [member] | Within 10 years [member]    
Disclosure of Detailed Information About Unrecognised Tax Losses [line Items]    
Gross Amount 0 0
Tax Effected 0 0
UK [member] | Within 20 years [member]    
Disclosure of Detailed Information About Unrecognised Tax Losses [line Items]    
Gross Amount 0 0
Tax Effected 0 0
UK [member] | Available indefinitely [member]    
Disclosure of Detailed Information About Unrecognised Tax Losses [line Items]    
Gross Amount 167 167
Tax Effected 42 42
US [member]    
Disclosure of Detailed Information About Unrecognised Tax Losses [line Items]    
Gross Amount 567 616
Tax Effected 94 101
US [member] | Within 10 years [member]    
Disclosure of Detailed Information About Unrecognised Tax Losses [line Items]    
Gross Amount 411 443
Tax Effected 86 92
US [member] | Within 20 years [member]    
Disclosure of Detailed Information About Unrecognised Tax Losses [line Items]    
Gross Amount 117 135
Tax Effected 6 7
US [member] | Available indefinitely [member]    
Disclosure of Detailed Information About Unrecognised Tax Losses [line Items]    
Gross Amount 39 38
Tax Effected 2 2
Other [Member]    
Disclosure of Detailed Information About Unrecognised Tax Losses [line Items]    
Gross Amount 162 182
Tax Effected 50 57
Other [Member] | Within 10 years [member]    
Disclosure of Detailed Information About Unrecognised Tax Losses [line Items]    
Gross Amount 14 21
Tax Effected 3 5
Other [Member] | Within 20 years [member]    
Disclosure of Detailed Information About Unrecognised Tax Losses [line Items]    
Gross Amount 0 0
Tax Effected 0 0
Other [Member] | Available indefinitely [member]    
Disclosure of Detailed Information About Unrecognised Tax Losses [line Items]    
Gross Amount 148 161
Tax Effected £ 47 £ 52