Hi,
The general rule is that any rental business loss is automatically carried forward and set off against rental business profits of the following year (ITA07/S118 and S119). This is before utilising your personal allowance, any unused property losses can then be carried forward to future years.
PIM4210 - Losses: setting losses against future profits
With regards to tax relief for finance costs, the reduction is the basic rate value (currently 20%) of the adjusted total income - the income (after losses and reliefs, and excluding savings and dividends income) that exceeds your personal allowance.
Tax relief for residential landlords: how it's worked out
Thank you.