When BR is normal
BR often appears on a second job, a short extra job, or a pension where HMRC expects your main Personal Allowance to sit against your main income source instead.
When BR may be wrong
- You only have one job and no other taxable income source is using your allowance.
- You changed jobs and payroll does not yet have the right P45 or starter-checklist information.
- You expected 1257L or another cumulative code and the allowance never moved across.
Best next pages
Use Second Job and Tax Code Calculator if you really do have two incomes, or Emergency Tax and Refund Checker if the code looks temporary or new-job related.
Official source: GOV.UK what your tax code means.