CH154120 | Penalties for Failure to Pay on Time: Rules for specific taxes: Registered Pension Scheme (RPS): 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 the income tax due under the Registered Pension Scheme by the penalty date, see CH154010.

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

See the example at CH154180.