Tax - Schedule of tax included within the statement of financial position (Details) - GBP (£) £ in Millions	12 Months Ended
Mar. 31, 2024	Mar. 31, 2023
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	£ 7,181	£ 6,765
Exchange adjustments and other	(110)	255
Charged/(credited) to income statement	467	509
Charged/(credited) to other comprehensive income and equity	7	(349)
Disposals	(2)	1
Reclassification to held for sale	(24)
Deferred tax liability (asset) end of period	7,519	7,181
Impact of IAS 12 amendment
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	0
Deferred tax liability (asset) end of period		0
Opening balance after adjustment
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	7,181
Deferred tax liability (asset) end of period		7,181
Regulatory licenses
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	429	429
Exchange adjustments and other	0	0
Charged/(credited) to income statement	0	0
Charged/(credited) to other comprehensive income and equity	0	0
Disposals	0	0
Reclassification to held for sale	0
Deferred tax liability (asset) end of period	429	429
Regulatory licenses \| Impact of IAS 12 amendment
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	0
Deferred tax liability (asset) end of period		0
Regulatory licenses \| Opening balance after adjustment
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	429
Deferred tax liability (asset) end of period		429
Accelerated tax depreciation
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	8,213	7,710
Exchange adjustments and other	(132)	357
Charged/(credited) to income statement	720	145
Charged/(credited) to other comprehensive income and equity	0	0
Disposals	(2)	1
Reclassification to held for sale	(12)
Deferred tax liability (asset) end of period	8,816	8,213
Accelerated tax depreciation \| Impact of IAS 12 amendment
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	29
Deferred tax liability (asset) end of period		29
Accelerated tax depreciation \| Opening balance after adjustment
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	8,242
Deferred tax liability (asset) end of period		8,242
Share- based payments
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	(21)	(18)
Exchange adjustments and other	0	(2)
Charged/(credited) to income statement	(5)	(2)
Charged/(credited) to other comprehensive income and equity	0	1
Disposals	0	0
Reclassification to held for sale	1
Deferred tax liability (asset) end of period	(25)	(21)
Share- based payments \| Impact of IAS 12 amendment
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	0
Deferred tax liability (asset) end of period		0
Share- based payments \| Opening balance after adjustment
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	(21)
Deferred tax liability (asset) end of period		(21)
Pensions and other post- retirement benefits
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	490	775
Exchange adjustments and other	(1)	8
Charged/(credited) to income statement	26	51
Charged/(credited) to other comprehensive income and equity	(50)	(344)
Disposals	0	0
Reclassification to held for sale	(4)
Deferred tax liability (asset) end of period	461	490
Pensions and other post- retirement benefits \| Impact of IAS 12 amendment
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	0
Deferred tax liability (asset) end of period		0
Pensions and other post- retirement benefits \| Opening balance after adjustment
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	490
Deferred tax liability (asset) end of period		490
Financial instruments
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	(370)	(301)
Exchange adjustments and other		8
Charged/(credited) to income statement	38	(71)
Charged/(credited) to other comprehensive income and equity	57	(6)
Disposals		0
Reclassification to held for sale	0
Deferred tax liability (asset) end of period	(275)	(370)
Financial instruments \| Impact of IAS 12 amendment
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	0
Deferred tax liability (asset) end of period		0
Financial instruments \| Opening balance after adjustment
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	(370)
Deferred tax liability (asset) end of period		(370)
Other net temporary differences
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	(1,560)	(1,830)
Exchange adjustments and other	23	(116)
Charged/(credited) to income statement	(312)	386
Charged/(credited) to other comprehensive income and equity	0	0
Disposals	0	0
Reclassification to held for sale	(9)
Deferred tax liability (asset) end of period	(1,887)	(1,560)
Deferred tax assets	1,887	1,560
Other net temporary differences \| Impact of IAS 12 amendment
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	(29)
Deferred tax liability (asset) end of period		(29)
Other net temporary differences \| Opening balance after adjustment
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax liability (asset) beginning of period	(1,589)
Deferred tax liability (asset) end of period		(1,589)
Net operating losses
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax assets	184	47
Lease liabilities
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax assets	575	511
Environmental provisions
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax assets	646	503
Allowance for credit losses
Reconciliation of changes in deferred tax liability (asset) [abstract]
Deferred tax assets	£ 150	£ 148

The amounts of income taxes recoverable in future periods in respect of: (a) deductible temporary differences; (b) the carryforward of unused tax losses; and (c) the carryforward of unused tax credits. [Refer: Temporary differences [member]; Unused tax credits [member]; Unused tax losses [member]]

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The amount of tax expense or income relating to changes in deferred tax liabilities and deferred tax assets, recognised in profit or loss. [Refer: Deferred tax assets; Deferred tax expense (income); Deferred tax liabilities]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 12
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The amount of deferred tax liabilities or assets. [Refer: Deferred tax liabilities; Deferred tax assets]

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The decrease in deferred tax liability (asset) resulting from the loss of control of a subsidiary. [Refer: Deferred tax liability (asset)]

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The increase (decrease) in deferred tax liability (asset) resulting from the net exchange differences arising when the financial statements are translated from the functional currency into a different presentation currency, including the translation of a foreign operation into the presentation currency of the reporting entity. [Refer: Deferred tax liability (asset)]

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Deferred Tax Expense (Income) Recognised In Other Comprehensive Income Or Charges (Credited) Directly To Equity

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No definition available.

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Increase (Decrease) Through Reclassification To Held For Sale, Deferred Tax Liability (Asset)

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