I have 2 properties, both with mortgages. After living there initially for 2 years, my first is now being let out, managed by an letting agent, still on a residential tracker interest only mortgage. I phoned the mortgage company before I did this and they gave me permission to let it at a cost of £95 a year. Given it's interest only, I am using the extra income to invest/save so I can pay some/most of it off in the event of BoE interest rate rises. My second is a recently purchased residential repayment mortgage.
My feelings are that if you can live there initially - working out the finances with your dad + a proper residential mortgage, you will be in a far far better position in a year or twos time to either get permission to let on your existing mortgage or get a proper BTL mortgage (you might have a little more equity to play with if it increases in value).