Derivative financial instruments - Disclosure of effect on the reserves as a result of hedge accounting (Details) - GBP (£) £ in Millions	12 Months Ended
Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Cash flow hedging reserve
Reserve of cash flow hedges, beginning of period	£ (2,448)	£ (2,895)
Currency translation movements	24	20
Hedging (losses)/gains for the year	2,630	(1,369)
Amounts reclassified in relation to cash flows affecting profit or loss	(506)	1,950	£ 1,158
Tax	(607)	(154)	(1,002)
Reserve of cash flow hedges, end of period	(907)	(2,448)	(2,895)
Currency translation reserve
Currency translation reserve, beginning of period	3,690	3,783
Currency translation movements	(1,640)	(194)
Hedging (losses)/gains for the year	499	50
Amounts reclassified in relation to cash flows affecting profit or loss	34	1
Tax	(49)	50	33
Currency translation reserve, end of period	£ 2,534	£ 3,690	£ 3,783

Gains (Losses) On Cash Flow Hedges Excluding Exchange Differences, Before Tax

No definition available.

Name:	bbplc_GainsLossesOnCashFlowHedgesExcludingExchangeDifferencesBeforeTax
Namespace Prefix:	bbplc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Gains (Losses) On Exchange Differences On Cash Flow Hedges, Before Tax

+ References

No definition available.

+ Details

Name:	bbplc_GainsLossesOnExchangeDifferencesOnCashFlowHedgesBeforeTax
Namespace Prefix:	bbplc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Reconciliation Of Currency Translation Reserve

+ References

No definition available.

+ Details

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

- Definition

Reconciliation Of Hedging Reserve

+ References

No definition available.

+ Details

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

- Definition

The gains (losses) recognised in other comprehensive income on exchange differences on the translation of financial statements of foreign operations, before tax, before reclassification adjustments. [Refer: Other comprehensive income]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Note Expiry date 2027-01-01
-Name IAS
-Number 1
-IssueDate 2025-01-01
-Paragraph 91
-Subparagraph b
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=1&code=ifrs-tx-2025-en-b&anchor=para_91_b&doctype=Standard
-URIDate 2025-03-27

+ Details

Name:	ifrs-full_GainsLossesOnExchangeDifferencesOnTranslationBeforeTax
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The gains (losses) recognised in other comprehensive income on hedges of net investments in foreign operations, before tax, before reclassification adjustments. [Refer: Other comprehensive income]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 39
-IssueDate 2025-01-01
-Paragraph 102
-Subparagraph a
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=39&code=ifrs-tx-2025-en-r&anchor=para_102_a&doctype=Standard
-URIDate 2025-03-27

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 9
-IssueDate 2025-01-01
-Paragraph 6.5.13
-Subparagraph a
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=9&code=ifrs-tx-2025-en-r&anchor=para_6.5.13_a&doctype=Standard
-URIDate 2025-03-27

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Note Expiry date 2027-01-01
-Name IAS
-Number 1
-IssueDate 2025-01-01
-Paragraph 91
-Subparagraph b
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=1&code=ifrs-tx-2025-en-b&anchor=para_91_b&doctype=Standard
-URIDate 2025-03-27

+ Details

Name:	ifrs-full_GainsLossesOnHedgesOfNetInvestmentsInForeignOperationsBeforeTax
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of income tax relating to amounts recognised in other comprehensive income in relation to cash flow hedges. [Refer: Cash flow hedges [member]; Other comprehensive income]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 12
-IssueDate 2025-01-01
-Paragraph 81
-Subparagraph ab
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=12&code=ifrs-tx-2025-en-r&anchor=para_81_ab&doctype=Standard
-URIDate 2025-03-27

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Note Expiry date 2027-01-01
-Name IAS
-Number 1
-IssueDate 2025-01-01
-Paragraph 90
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=1&code=ifrs-tx-2025-en-b&anchor=para_90&doctype=Standard
-URIDate 2025-03-27

+ Details

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

- Definition

The amount of income tax relating to amounts recognised in other comprehensive income in relation to exchange differences on the translation of financial statements of foreign operations. [Refer: Other comprehensive income; Reserve of exchange differences on translation]

+ References

+ Details

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

- Definition

The amount of reclassification adjustments related to cash flow hedges, before tax. Reclassification adjustments are amounts reclassified to profit (loss) in the current period that were recognised in other comprehensive income in the current or previous periods. [Refer: Cash flow hedges [member]; Other comprehensive income]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Note Expiry date 2027-01-01
-Name IAS
-Number 1
-IssueDate 2025-01-01
-Paragraph 92
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=1&code=ifrs-tx-2025-en-b&anchor=para_92&doctype=Standard
-URIDate 2025-03-27

+ Details

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

- Definition

The amount of reclassification adjustments related to hedges of net investments in foreign operations, before tax. Reclassification adjustments are amounts reclassified to profit (loss) in the current period that were recognised in other comprehensive income in the current or previous periods. [Refer: Other comprehensive income]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 39
-IssueDate 2025-01-01
-Paragraph 102
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=39&code=ifrs-tx-2025-en-r&anchor=para_102&doctype=Standard
-URIDate 2025-03-27

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 9
-IssueDate 2025-01-01
-Paragraph 6.5.14
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=9&code=ifrs-tx-2025-en-r&anchor=para_6.5.14&doctype=Standard
-URIDate 2025-03-27

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Note Expiry date 2027-01-01
-Name IAS
-Number 1
-IssueDate 2025-01-01
-Paragraph 92
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=1&code=ifrs-tx-2025-en-b&anchor=para_92&doctype=Standard
-URIDate 2025-03-27

+ Details

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

- Definition

A component of equity representing the accumulated portion of gain (loss) on a hedging instrument that is determined to be an effective hedge for cash flow hedges. [Refer: Cash flow hedges [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 9
-IssueDate 2025-01-01
-Paragraph 6.5.11
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=9&code=ifrs-tx-2025-en-r&anchor=para_6.5.11&doctype=Standard
-URIDate 2025-03-27

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Note Expiry date 2027-01-01
-Name IAS
-Number 1
-IssueDate 2025-01-01
-Paragraph 78
-Subparagraph e
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=1&code=ifrs-tx-2025-en-b&anchor=para_78_e&doctype=Standard
-URIDate 2025-03-27

+ Details

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

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2025-01-01
-Paragraph 24B
-Subparagraph b
-Clause ii
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2025-en-r&anchor=para_24B_b_ii&doctype=Standard
-URIDate 2025-03-27

+ Details

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