PROVISIONS - Additional Information (Details) £ in Millions	3 Months Ended	6 Months Ended
PROVISIONS - Additional Information (Details) £ in Millions	Mar. 31, 2026 GBP (£) branch	Jun. 30, 2026 GBP (£)	Jun. 30, 2025 GBP (£)	Apr. 01, 2026 branch cafe employee local	Dec. 31, 2025 GBP (£)	Mar. 31, 2025 branch
Disclosure of other provisions [line items]
Provisions		£ 950			£ 683
Additional provisions		505
Branch Transformation Project
Disclosure of other provisions [line items]
Number of branch closures \| branch	14			44		95
Number of branches \| branch				305
Number of work cafes with support provided \| cafe				6
Number of santander locals \| local				111
Branch Transformation Project \| Forecast
Disclosure of other provisions [line items]
Number of employees at risk for redundancy \| employee				287
Branch Transformation Project \| Full service branches
Disclosure of other provisions [line items]
Number of branches \| branch				244
Branch Transformation Project \| Counter free branches
Disclosure of other provisions [line items]
Number of branches \| branch				19
Branch Transformation Project \| Reduced hours branches
Disclosure of other provisions [line items]
Number of branches \| branch				36
Customer remediation - Motor
Disclosure of other provisions [line items]
Provisions	£ 633	623			461
Additional provisions		179
Restructuring
Disclosure of other provisions [line items]
Provisions		72			21
Additional provisions		95
Restructuring \| Branch Transformation Project
Disclosure of other provisions [line items]
Additional provisions	30
Restructuring \| Branch Transformation Project \| Redundancy provision
Disclosure of other provisions [line items]
Additional provisions	10
Restructuring \| Branch Transformation Project \| Property provisions
Disclosure of other provisions [line items]
Additional provisions	9
Restructuring \| Branch Transformation Project \| Impairment of property
Disclosure of other provisions [line items]
Additional provisions	£ 11
Regulatory levies and fees
Disclosure of other provisions [line items]
Provisions		71			£ 6
Additional provisions		83	£ 93
Regulatory levies and fees \| Bank of England
Disclosure of other provisions [line items]
Additional provisions		40	39
Operational risk provisions
Disclosure of other provisions [line items]
Additional provisions		48	42
Fraud losses
Disclosure of other provisions [line items]
Additional provisions		£ 33	£ 25