v3.25.4
Classification of Financial Instruments - Schedule of Analysis of the Movements in Level 3 Fair Value Remeasurements (Detail) - GBP (£)
£ in Millions
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Disclosure of financial assets [line items]    
At beginning of year £ 141 £ 143
Exchange differences (7) 2
Acquisition of investments and other receivable 5 9
Reclassification out of level 3   (6)
At end of year 125 141
Financial assets at fair value through other comprehensive income, category [member]    
Disclosure of financial assets [line items]    
At beginning of year 28  
Fair value movements (7) (2)
At end of year 24 28
Financial assets at fair value through profit or loss, category [member]    
Disclosure of financial assets [line items]    
Fair value movements (7) (11)
Level 3 of fair value hierarchy [member]    
Disclosure of financial assets [line items]    
At beginning of year 147 155
Exchange differences (7) 2
Acquisition of investments and other receivable 5 9
Repayments (1) 0
Reclassification out of level 3 0 (6)
At end of year 137 147
Level 3 of fair value hierarchy [member] | Financial assets at fair value through other comprehensive income, category [member]    
Disclosure of financial assets [line items]    
Fair value movements (2) (2)
Level 3 of fair value hierarchy [member] | Financial assets at fair value through profit or loss, category [member]    
Disclosure of financial assets [line items]    
Fair value movements (5) (11)
Level 3 of fair value hierarchy [member] | Other receivable [member]    
Disclosure of financial assets [line items]    
At beginning of year 12  
Exchange differences 0  
Acquisition of investments and other receivable 0  
Repayments (1)  
Reclassification out of level 3 0  
At end of year 13 12
Level 3 of fair value hierarchy [member] | Other receivable [member] | Financial assets at fair value through other comprehensive income, category [member]    
Disclosure of financial assets [line items]    
Fair value movements 0  
Level 3 of fair value hierarchy [member] | Other receivable [member] | Financial assets at fair value through profit or loss, category [member]    
Disclosure of financial assets [line items]    
Fair value movements 2  
Level 3 of fair value hierarchy [member] | Investments in unlisted securities [member]    
Disclosure of financial assets [line items]    
At beginning of year 135  
Exchange differences (7)  
Acquisition of investments and other receivable 5  
Repayments 0  
Reclassification out of level 3 0  
At end of year 124 £ 135
Level 3 of fair value hierarchy [member] | Investments in unlisted securities [member] | Financial assets at fair value through other comprehensive income, category [member]    
Disclosure of financial assets [line items]    
Fair value movements (2)  
Level 3 of fair value hierarchy [member] | Investments in unlisted securities [member] | Financial assets at fair value through profit or loss, category [member]    
Disclosure of financial assets [line items]    
Fair value movements £ (7)