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Financial instruments and risk management - Interest rate risk - Narrative (Details) - GBP (£)
£ in Millions
Jun. 30, 2023
Jun. 30, 2022
Disclosure of detailed information about financial instruments [line items]    
Borrowings percentage 100.00% 100.00%
Fair value hedges | Interest rate risk    
Disclosure of detailed information about financial instruments [line items]    
Notional amounts £ 3,999 £ 4,444
USD LIBOR | Fair value hedges | Interest rate risk    
Disclosure of detailed information about financial instruments [line items]    
Notional amounts £ 2,063 £ 2,893
Fixed rate    
Disclosure of detailed information about financial instruments [line items]    
Borrowings percentage 77.00% 78.00%
Minimum | Fixed rate    
Disclosure of detailed information about financial instruments [line items]    
Borrowings percentage 40.00%  
Maximum | Fixed rate    
Disclosure of detailed information about financial instruments [line items]    
Borrowings percentage 90.00%