If the loan is from a family member and they are a non taxpayer they won’t be able to use form R85 but they will still be able to claim the tax back either by contacting HMRC 0845 366 7850 or by filing a self assessment return
Interest payments are not subject to National Insurance and can be a tax efficient of recieving part of your income from your business.
The income will need to be reported on your self assessment return.
The interest charges will be tax deductable by the Company.
This will make the you ‘the lender’ a secured creditor but you may have to take a secondary position behind your bank or other lenders or if other lenders are already lending you may need their permission to register a charge.