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Financial instruments - Summary of gains and losses on the borrowings and related derivatives designated as fair value hedges (Details) - GBP (£)
£ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Disclosure Of Detailed Information About Hedged Items      
Borrowings £ (6,497) £ (6,730)  
US dollars      
Disclosure Of Detailed Information About Hedged Items      
Borrowings (2,459) (3,227)  
Euro      
Disclosure Of Detailed Information About Hedged Items      
Borrowings (3,973) (3,343)  
Fair value hedges      
Disclosure Of Detailed Information About Hedged Items      
Accumulated fair value hedge adjustment on hedged item at start of year 211 (28) £ (119)
Fair value movement gain/(loss) (83) 236 90
Redemption/close-out (16)    
Exchange gain/(loss) (8) 3 1
Accumulated fair value hedge adjustment on hedged item at end of year 104 211 (28)
Borrowings (2,471) (2,554) (2,161)
Accumulated fair value adjustment on hedging instrument at start of year (213) 35 119
Fair value movement gain/(loss) 81 (245) (83)
Redemption/close-out 16    
Exchange gain/(loss) 8 (3) (1)
Accumulated fair value adjustment on hedging instrument at end of year (108) (213) 35
Hedging instrument, assets     35
Hedging instrument, liabilities (108) (213)  
Accumulated fair value adjustment on hedged item and hedging instrument at start of year (2) 7  
Gain (loss) on change in fair value of hedged item and hedging instrument (2) (9) 7
Accumulated fair value adjustment on hedged item and hedging instrument at end of year (4) (2) 7
Borrowings net of related hedging instruments (2,579) (2,767) (2,126)
Fair value hedges | US dollars      
Disclosure Of Detailed Information About Hedged Items      
Accumulated fair value hedge adjustment on hedged item at start of year 141 (1) (36)
Fair value movement gain/(loss) (22) 140 35
Redemption/close-out (16)    
Exchange gain/(loss) (6) 2  
Accumulated fair value hedge adjustment on hedged item at end of year 97 141 (1)
Borrowings (871) (1,630) (1,221)
Accumulated fair value adjustment on hedging instrument at start of year (143) 8 36
Fair value movement gain/(loss) 21 (149) (28)
Redemption/close-out 16    
Exchange gain/(loss) 6 (2)  
Accumulated fair value adjustment on hedging instrument at end of year (100) (143) 8
Hedging instrument, assets     8
Hedging instrument, liabilities (100) (143)  
Accumulated fair value adjustment on hedged item and hedging instrument at start of year (2) 7  
Gain (loss) on change in fair value of hedged item and hedging instrument (1) (9) 7
Accumulated fair value adjustment on hedged item and hedging instrument at end of year (3) (2) 7
Borrowings net of related hedging instruments (971) (1,773) (1,213)
Fair value hedges | Euro      
Disclosure Of Detailed Information About Hedged Items      
Accumulated fair value hedge adjustment on hedged item at start of year 70 (27) (83)
Fair value movement gain/(loss) (61) 96 55
Exchange gain/(loss) (2) 1 1
Accumulated fair value hedge adjustment on hedged item at end of year 7 70 (27)
Borrowings (1,600) (924) (940)
Accumulated fair value adjustment on hedging instrument at start of year (70) 27 83
Fair value movement gain/(loss) 60 (96) (55)
Exchange gain/(loss) 2 (1) (1)
Accumulated fair value adjustment on hedging instrument at end of year (8) (70) 27
Hedging instrument, assets     27
Hedging instrument, liabilities (8) (70)  
Gain (loss) on change in fair value of hedged item and hedging instrument (1)    
Accumulated fair value adjustment on hedged item and hedging instrument at end of year (1)    
Borrowings net of related hedging instruments £ (1,608) £ (994) £ (913)