Thank you for your question..
Mortgage interest is a revenue expense and can only be offset against the rental income, I'm afraid. I am sorry if this is not the answer you were expecting.
Costs associated with the purchase of property (e.g. solicitor's cost, stamp duty etc) as opposed to financing the purchase can be added to the purchase price when calculating the capital gain for CGT purposes.
I hope this is helpful and answers your question.
If you have any other questions, please ask me before you rate my service – I’ll be happy to respond.