Hi Anna Evans,
There is no maximum allowable limit for relief for finance costs relating to rental property, as long as the loan was used 'wholly and exclusively' for the purchase of the property.
The loan interest and any loan arrangement fees will normally be allowable as a revenue expense.
From tax year 2017-18 over a period of 4 years, the tax relief that landlords of residential properties get for finance costs was calculated differently and so was gradually all restricted to the basic rate of income tax.
From 2020/21 tax relief is restricted to 20% of the finance costs.
More information can be found at
Property Income Manual PIM2052 - Deductions: interest: overview .
Thank you.