Financial instruments and risk management - Schedule of fluctuations in interest and exchange rates to vary from hypothetical amounts (Details) - GBP (£) £ in Millions	12 Months Ended
Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2021
Disclosure of nature and extent of risks arising from financial instruments [line items]
Impact on income statement gain/(loss)	£ 3,766	£ 3,338	£ 2,799
Total comprehensive gain/(loss)	£ 2,964	4,820	£ 1,934
Interest rate risk
Disclosure of nature and extent of risks arising from financial instruments [line items]
Sensitivity analysis for types of market risk, percentage of reasonably possible change in risk variable	0.50%
Interest rate risk \| 0.5% decrease in interest rates
Disclosure of nature and extent of risks arising from financial instruments [line items]
Sensitivity analysis for types of market risk, percentage of reasonably possible change in risk variable	(0.50%)
Impact on income statement gain/(loss)	£ 16	13
Total comprehensive gain/(loss)	£ 36	31
Interest rate risk \| 0.5% increase in interest rates
Disclosure of nature and extent of risks arising from financial instruments [line items]
Sensitivity analysis for types of market risk, percentage of reasonably possible change in risk variable	0.50%
Impact on income statement gain/(loss)	£ (16)	(13)
Total comprehensive gain/(loss)	£ (35)	(30)
Currency exchange risk
Disclosure of nature and extent of risks arising from financial instruments [line items]
Sensitivity analysis for types of market risk, percentage of reasonably possible change in risk variable	10.00%
Currency exchange risk \| 10% weakening of sterling
Disclosure of nature and extent of risks arising from financial instruments [line items]
Sensitivity analysis for types of market risk, percentage of reasonably possible change in risk variable	(10.00%)
Impact on income statement gain/(loss)	£ (45)	(33)
Total comprehensive gain/(loss)	£ (1,336)	(1,125)
Currency exchange risk \| 10% strengthening of sterling
Disclosure of nature and extent of risks arising from financial instruments [line items]
Sensitivity analysis for types of market risk, percentage of reasonably possible change in risk variable	10.00%
Impact on income statement gain/(loss)	£ 36	28
Total comprehensive gain/(loss)	£ 1,093	£ 922

Disclosure of nature and extent of risks arising from financial instruments [line items]