Hi,
Since the 2020 to 2021 tax year, interest on mortgage payments or residential finance costs are not allowable as expenses.
They are instead used as a reduction to your tax liability at 20%. For example, £1000 interest costs x 20% = £200. Your overall tax bill is then reduced by £200.
Regardless of your total income you are only allowed relief at 20%.
You can find guidance on how to work this out here:
Tax relief for residential landlords: how it's worked out
Thank you.