Dividends on ordinary shares and preference shares - Dividends on ordinary shares and preference shares (Details) - GBP (£) £ in Millions	6 Months Ended
Jun. 30, 2025	Dec. 31, 2024	Jun. 30, 2024
Dividends1 [Abstract]
Ordinary shares	£ 1,195	£ 930	£ 852
Preference shares	18	£ 20	21
Total	£ 1,213		£ 873

No definition available.

Name:	bbplc_Dividends1Abstract
Namespace Prefix:	bbplc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of dividends recognised as distributions to owners.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2024-01-01
-Paragraph 107
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=1&code=ifrs-tx-2024-en-r&anchor=para_107&doctype=Standard
-URIDate 2024-03-27

+ Details

Name:	ifrs-full_DividendsPaid
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of dividends paid by the entity on ordinary shares. [Refer: Ordinary shares [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 34
-IssueDate 2024-01-01
-Paragraph 16A
-Subparagraph f
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=34&code=ifrs-tx-2024-en-r&anchor=para_16A_f&doctype=Standard
-URIDate 2024-03-27

+ Details

Name:	ifrs-full_DividendsPaidOrdinaryShares
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of dividends paid by the entity on shares other than ordinary shares. [Refer: Ordinary shares [member]]

+ References

+ Details

Name:	ifrs-full_DividendsPaidOtherShares
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration