Remortgage currently finance-free holiday let; will loan interest be an allowable expense?

I own a commercially let holiday cottage outright. For many years it has been successfully let within HMRC rules. Self-assessment returns have been made each year and I have paid tax on a healthy profit. I need to raise cash for the construction of a property which may eventually become my primary residence. Is it allowable to obtain a loan against the value of the holiday let and to offset the interest paid against my rental income? Obviously, the balance of my tax return would change significantly and I would have intentionally put myself in an advantageous tax position.

Hi parallelcottages,
The re-mortgage would not be allowable against any future expenses.
Thank you.