I had a mate in a position like this and as long as you yourself have the qualifying funds and/or job to meet the threshhold it should not be an issue,the house owner(your dad) would need to send a covering letter with the title deeds to show he owns the house and you will not just be staying there short term and show you will have at least 1 bedroom for yourself and your fiance.It actually would be better for him to make up a small lease for you to say he is renting you the space,it would be much easier