CH155120 | Penalties for Failure to Pay on Time: Rules for specific taxes: Income Tax and Capital Gains Tax (IT & CGT): Penalties: Failure to pay all or part of amounts due - initial penalty

From HM Revenue & Customs · Compliance Handbook

A person incurs an initial penalty when they do not pay income tax or capital gains tax by the penalty date, see CH155010.

The amount of the initial penalty is 5% of the amount unpaid at the penalty date.

See the example at CH155180.