Financial risk management - Disclosure of effects of hedge accounting on financial position and year-to-date performance for each type of hedge (Details) £ in Millions	12 Months Ended
Mar. 31, 2024 GBP (£)	Mar. 31, 2023 GBP (£)
Foreign currency and interest rate risk \| Fair value hedges
Disclosure of detailed information about hedging instruments [line items]
Hedging instrument notional	(5,096)	(4,779)
Balance of fair value hedge adjustments in borrowings, Continuing hedges	£ 720	£ 789
Balance of fair value hedge adjustments in borrowings, Discontinued hedges	(35)	(43)
Change in value used for calculating ineffectiveness, Hedged item	40	398
Change in value used for calculating ineffectiveness, Hedging instrument	(22)	(351)
Hedge ineffectiveness	18	47
Foreign currency and interest rate risk \| Fair value hedges \| Impacted by Interest Rate Benchmark Reform amendments
Disclosure of detailed information about hedging instruments [line items]
Nominal amount of hedging instruments in hedging relationships to which amendments for interest rate benchmark reform are applied		859
Foreign currency and interest rate risk \| Fair value hedges \| Borrowings
Disclosure of detailed information about hedging instruments [line items]
Hedged borrowings	4,364	4,042
Foreign currency and interest rate risk \| Fair value hedges \| Borrowings \| Current liabilities
Disclosure of detailed information about hedging instruments [line items]
Hedged borrowings	271	511
Foreign currency and interest rate risk \| Fair value hedges \| Borrowings \| Non-current liabilities
Disclosure of detailed information about hedging instruments [line items]
Hedged borrowings	£ 4,093	£ 3,531
Foreign currency and interest rate risk \| Cash flow hedges
Disclosure of detailed information about hedging instruments [line items]
Hedging instrument notional	(9,892)	(9,357)
Balance in cash flow hedge reserve, Continuing hedges	£ 154	£ (73)
Balance in cash flow hedge reserve, Discontinued hedges	0	0
Change in value used for calculating ineffectiveness, Hedged item	(18)	149
Change in value used for calculating ineffectiveness, Hedging instrument	3	(154)
Hedge ineffectiveness	£ (15)	£ (5)
Currency risk \| Cash flow hedges
Disclosure of detailed information about hedging instruments [line items]
Hedging instrument notional	(2,039)	(537)
Balance in cash flow hedge reserve, Continuing hedges	£ (31)	£ (3)
Balance in cash flow hedge reserve, Discontinued hedges	0	0
Change in value used for calculating ineffectiveness, Hedged item	28	(35)
Change in value used for calculating ineffectiveness, Hedging instrument	(28)	35
Hedge ineffectiveness	£ 0	£ 0
Currency risk \| Net investment hedges
Disclosure of detailed information about hedging instruments [line items]
Hedging instrument notional	(2,999)	(3,095)
Balance in translation reserve, Continuing hedges	£ 40	£ (129)
Balance in translation reserve, Discontinued hedges	(2,564)	(2,457)
Change in value used for calculating ineffectiveness, Hedged item	(62)	198
Change in value used for calculating ineffectiveness, Hedging instrument	62	(198)
Hedge ineffectiveness	£ 0	£ 0

The accumulated amount of fair value hedge adjustment on a hedged item that is included in the carrying amount of the hedged item, recognised in the statement of financial position as a liability. [Refer: Hedged items [member]]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2023-01-01
-Paragraph 24B
-Subparagraph a
-Clause ii
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-URIDate 2023-03-23

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The accumulated amount of fair value hedge adjustment remaining in the statement of financial position for a hedged item that is a financial liability measured at amortised cost and has ceased to be adjusted for hedging gains and losses. [Refer: Financial liabilities at amortised cost; Hedged items [member]]

+ References

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Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with members defined in one or many axes of the table.

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No definition available.

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The gain (loss) on the change in fair value of the hedged item used as a basis for recognising hedge ineffectiveness. Hedge ineffectiveness is the extent to which the changes in the fair value or the cash flows of the hedging instrument are greater or less than those on the hedged item.

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The gain (loss) on the change in fair value of the hedging instrument used as a basis for recognising hedge ineffectiveness. Hedge ineffectiveness is the extent to which the changes in the fair value or the cash flows of the hedging instrument are greater or less than those on the hedged item.

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The gain (loss) on hedge ineffectiveness. Hedge ineffectiveness is the extent to which the changes in the fair value or the cash flows of the hedging instrument are greater or less than those on the hedged item.

+ References

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The amount of a hedged item, recognised as a liability. [Refer: Hedged items [member]]

+ References

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The nominal amount of a hedging instrument. [Refer: Hedging instruments [member]]

+ References

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The nominal amount of the hedging instruments in hedging relationships to which the entity is applying the exceptions in the scope of the amendments for interest rate benchmark reform. [Refer: Nominal amount of hedging instrument]

+ References

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A component of equity representing the reserve of cash flow hedges in relation to continuing hedges. [Refer: Reserve of cash flow hedges]

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-Name IFRS
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A component of equity representing the reserve of cash flow hedges in relation to hedging relationships for which hedge accounting is no longer applied. [Refer: Reserve of cash flow hedges]

+ References

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A component of equity representing the reserve of exchange differences on translation in relation to continuing hedges. [Refer: Reserve of exchange differences on translation]

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A component of equity representing the reserve of exchange differences on translation in relation to hedging relationships for which hedge accounting is no longer applied. [Refer: Reserve of exchange differences on translation]

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