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Net borrowings - Bonds issued and repaid (Details) - GBP (£)
£ in Millions
 12 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2019
Schedule of bonds issued and repaid [line items]      
Repaid £ (1,247) £ (820) £ (1,168)
Bonds issued and repaid (216) 4,368 1,598
€ denominated      
Schedule of bonds issued and repaid [line items]      
Issued 636 1,594 2,270
Repaid (696) 0 (1,168)
£ denominated      
Schedule of bonds issued and repaid [line items]      
Issued 395 298 496
US$ denominated      
Schedule of bonds issued and repaid [line items]      
Issued 0 3,296 0
Repaid £ (551) £ (820) £ 0