I work in this area.
unless you meet every requirement exactly your claim will definitely be refused.
they are so strict on this you need to be very through and tick every single box.
you need a proper legal assured short hold tenancy agreement.
you need to look at what the market rate for rent is in the area for properties of that size and the rent must fall within a certain threshold, usually it’s 10-15%.
it can not just be the cost of the mortgage/loan if that falls significantly below the market rate as that will trigger something that will immediately reject the claim.
there must be all the appropriate certificates, gas, energy efficiency, electrics. Etc.
and landlord and buildings insurance.
and most importantly you dad must be registered to do a self assessment tax return for the rental income, because even if he is making £0 profit and it’s all going on the loan/mortgage he will still have to pay tax on the rental income.
this is something they check 100% of the time as it’s the biggest indicator that the tenancy is not 100% above board.
They will also check when they do the annual review that it is being declared appropriately,
And if it isn’t they can decide you claimed fraudulently and come after you for all the money back.