Hi,
Prior to 2017 to 2018 the loan interest would have been allowable as a revenue expense. However, from 2017 to 2018, over a period of 4 years, the tax relief that landlords of residential properties get for finance costs was calculated differently and was gradually restricted to the basic rate of Iincome Tax. You can see guidance, including examples, here:
Tax relief for residential landlords: how it's worked out
From 2020 to 2021 onwards to claim relief for residential property finance costs you need to enter the costs claimed for at box 44 on the property page of your Self Assessment tax reurn.
Thank you.