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Financial instruments and risk management - Average monthly net borrowings and effective interest rate (Details)
€ in Millions, £ in Millions, $ in Millions
Jun. 30, 2021
EUR (€)
Jun. 30, 2021
GBP (£)
Jun. 30, 2021
USD ($)
Jun. 30, 2020
GBP (£)
Jun. 30, 2019
GBP (£)
Disclosure of average monthly borrowings and effective interest rate [line items]          
Average monthly net borrowings   £ 12,109   £ 13,246 £ 11,277
Effective interest rate 2.70% 2.70% 2.70% 2.60% 2.40%
Notional amounts € 900   $ 1,000    
Fair value hedges | Interest rate risk          
Disclosure of average monthly borrowings and effective interest rate [line items]          
Notional amounts   £ 4,646   £ 6,092  
USD LIBOR | Fair value hedges | Interest rate risk          
Disclosure of average monthly borrowings and effective interest rate [line items]          
Notional amounts   2,338   3,156  
Weighted average          
Disclosure of average monthly borrowings and effective interest rate [line items]          
Average monthly net borrowings   £ 12,702   £ 12,708 £ 10,393