v3.25.4
Classification of Financial Instruments - Schedule of Classification of Each Class of Financial Assets and their Carrying Values (Detail) - GBP (£)
£ in Millions
Dec. 31, 2025
Dec. 31, 2024
Disclosure of financial assets [line items]    
Financial assets £ 1,217 £ 1,515
Investments in listed and unlisted securities [member]    
Disclosure of financial assets [line items]    
Financial assets 125 141
Cash and cash equivalents [member]    
Disclosure of financial assets [line items]    
Financial assets 333 543
Derivative financial instruments [member]    
Disclosure of financial assets [line items]    
Financial assets 16 51
Trade receivables [member]    
Disclosure of financial assets [line items]    
Financial assets 661 614
Contract Assets Unbilled [Member]    
Disclosure of financial assets [line items]    
Financial assets   71
Investment in finance lease receivable [member]    
Disclosure of financial assets [line items]    
Financial assets 66 83
Other receivable [member]    
Disclosure of financial assets [line items]    
Financial assets 16 12
Financial assets at fair value [member] | FVOCI [member]    
Disclosure of financial assets [line items]    
Financial assets 24 28
Financial assets at fair value [member] | FVOCI [member] | Investments in listed and unlisted securities [member]    
Disclosure of financial assets [line items]    
Financial assets 24 28
Financial assets at fair value [member] | FVOCI [member] | Cash and cash equivalents [member]    
Disclosure of financial assets [line items]    
Financial assets 0 0
Financial assets at fair value [member] | FVOCI [member] | Derivative financial instruments [member]    
Disclosure of financial assets [line items]    
Financial assets 0 0
Financial assets at fair value [member] | FVOCI [member] | Trade receivables [member]    
Disclosure of financial assets [line items]    
Financial assets 0 0
Financial assets at fair value [member] | FVOCI [member] | Contract Assets Unbilled [Member]    
Disclosure of financial assets [line items]    
Financial assets   0
Financial assets at fair value [member] | FVOCI [member] | Investment in finance lease receivable [member]    
Disclosure of financial assets [line items]    
Financial assets 0 0
Financial assets at fair value [member] | FVOCI [member] | Other receivable [member]    
Disclosure of financial assets [line items]    
Financial assets 0 0
Financial assets at fair value [member] | FVTPL [member]    
Disclosure of financial assets [line items]    
Financial assets 142 217
Financial assets at fair value [member] | FVTPL [member] | Investments in listed and unlisted securities [member]    
Disclosure of financial assets [line items]    
Financial assets 101 113
Financial assets at fair value [member] | FVTPL [member] | Cash and cash equivalents [member]    
Disclosure of financial assets [line items]    
Financial assets 11 62
Financial assets at fair value [member] | FVTPL [member] | Derivative financial instruments [member]    
Disclosure of financial assets [line items]    
Financial assets 14 30
Financial assets at fair value [member] | FVTPL [member] | Trade receivables [member]    
Disclosure of financial assets [line items]    
Financial assets 0 0
Financial assets at fair value [member] | FVTPL [member] | Contract Assets Unbilled [Member]    
Disclosure of financial assets [line items]    
Financial assets   0
Financial assets at fair value [member] | FVTPL [member] | Investment in finance lease receivable [member]    
Disclosure of financial assets [line items]    
Financial assets 0 0
Financial assets at fair value [member] | FVTPL [member] | Other receivable [member]    
Disclosure of financial assets [line items]    
Financial assets 16 12
Financial assets at fair value [member] | Fair value - hedging instrument [member]    
Disclosure of financial assets [line items]    
Financial assets 2 21
Financial assets at fair value [member] | Fair value - hedging instrument [member] | Investments in listed and unlisted securities [member]    
Disclosure of financial assets [line items]    
Financial assets 0 0
Financial assets at fair value [member] | Fair value - hedging instrument [member] | Cash and cash equivalents [member]    
Disclosure of financial assets [line items]    
Financial assets 0 0
Financial assets at fair value [member] | Fair value - hedging instrument [member] | Derivative financial instruments [member]    
Disclosure of financial assets [line items]    
Financial assets 2 21
Financial assets at fair value [member] | Fair value - hedging instrument [member] | Trade receivables [member]    
Disclosure of financial assets [line items]    
Financial assets 0 0
Financial assets at fair value [member] | Fair value - hedging instrument [member] | Contract Assets Unbilled [Member]    
Disclosure of financial assets [line items]    
Financial assets   0
Financial assets at fair value [member] | Fair value - hedging instrument [member] | Investment in finance lease receivable [member]    
Disclosure of financial assets [line items]    
Financial assets 0 0
Financial assets at fair value [member] | Fair value - hedging instrument [member] | Other receivable [member]    
Disclosure of financial assets [line items]    
Financial assets 0 0
Financial assets at amortised cost [member] | Contract Assets Unbilled [Member]    
Disclosure of financial assets [line items]    
Financial assets   71
Financial assets at amortised cost [member] | Financial assets, class [member]    
Disclosure of financial assets [line items]    
Financial assets 1,049 1,249
Financial assets at amortised cost [member] | Financial assets, class [member] | Investments in listed and unlisted securities [member]    
Disclosure of financial assets [line items]    
Financial assets 0 0
Financial assets at amortised cost [member] | Financial assets, class [member] | Cash and cash equivalents [member]    
Disclosure of financial assets [line items]    
Financial assets 322 481
Financial assets at amortised cost [member] | Financial assets, class [member] | Derivative financial instruments [member]    
Disclosure of financial assets [line items]    
Financial assets 0 0
Financial assets at amortised cost [member] | Financial assets, class [member] | Trade receivables [member]    
Disclosure of financial assets [line items]    
Financial assets 661 614
Financial assets at amortised cost [member] | Financial assets, class [member] | Investment in finance lease receivable [member]    
Disclosure of financial assets [line items]    
Financial assets 66 83
Financial assets at amortised cost [member] | Financial assets, class [member] | Other receivable [member]    
Disclosure of financial assets [line items]    
Financial assets £ 0 £ 0