Payments on account
Advance payments towards next year's Self Assessment bill.
If your last bill was over £1,000 (and less than 80% was taken at source), HMRC asks for two advance payments, each half of last year's bill, on 31 January and 31 July. It's why the first January after a good year can be large.
General information about UK rules as of October 2026, not tax or legal advice.