Contract liabilities - Schedule of significant changes in contract assets and contract liabilities (Details) - GBP (£) £ in Millions	12 Months Ended
Mar. 31, 2026	Mar. 31, 2025
Increase (Decrease) in Contract Liabilities [Roll Forward]
As at 1 April	£ 2,514	£ 2,246
Exchange adjustments	(28)	(28)
Revenue recognised that was included in the contract liability balance at the beginning of the period	(97)	(129)
Increases due to cash received, excluding amounts recognised as revenue during the period	420	425
At 31 March	£ 2,809	£ 2,514

The amount of an entity’s obligation to transfer goods or services to a customer for which the entity has received consideration (or the amount is due) from the customer.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 15
-IssueDate 2025-01-01
-Paragraph 105
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=15&code=ifrs-tx-2025-en-r&anchor=para_105&doctype=Standard
-URIDate 2025-03-27

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 15
-IssueDate 2025-01-01
-Paragraph 116
-Subparagraph a
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=15&code=ifrs-tx-2025-en-r&anchor=para_116_a&doctype=Standard
-URIDate 2025-03-27

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Note Effective 2027-01-01
-Name IFRS
-Number 19
-IssueDate 2025-01-01
-Paragraph 105
-Subparagraph a
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=19&code=ifrs-tx-2025-en-r&anchor=para_105_a&doctype=Standard
-URIDate 2025-03-27

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Note Effective 2027-01-01
-Name IFRS
-Number 19
-IssueDate 2025-01-01
-Paragraph 105
-Subparagraph a
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=19&code=ifrs-tx-2025-en-r&anchor=para_105_a&doctype=Standard
-URIDate 2025-03-27

Name:	ifrs-full_ContractLiabilities
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant