Any and all loan interest repayments on a residential property wholly or partly owned by a private UK taxpayer which is let on a commercial basis (in other words, for a rental amount).
So if you rent out a property that has a mortgage (or any other type of loan) secured against the property, and pay your income tax by Self-Assessment, HMRC will add your loan interest payments to your profits to reach a taxable income figure.
For more guidance and examples from HMRC, please click the link below:
HMRC Guidance from gov.uk