Expense Apportionment for Partially Rented Second Home

I am seeking clarification regarding the tax treatment of a second home I own, which is not my main residence. I visit this property occasionally for work purposes and rent out two of its three rooms. When I visit, I stay in the remaining room. Could you confirm whether I can apportion expenses (such as bills and mortgage interest) between personal and rental use? For example, since two out of the three rooms are rented out, would I be able to allocate two-thirds of these expenses (including mortgage interest) to the rental activity? Additionally, does the fact that the mortgage is residential (and not a buy-to-let) affect this apportionment, or is it irrelevant? I would appreciate any relevant resources or links to further information. Regards 

Hi,
You may apportion expenses that are wholly and exclusively for use within the rental business. You can see the guidance here:
PIM2100 - Deductions: main types of expense: expenses for own home
Thank you.