When I was looking at the help to buy ISA, there are a few conditions which may be a problem. The terms say you must live there, do not rent it out and it will be the only property you own. How this is enforced I’m not sure!
If you are taking a residential mortgage but letting out rooms then should the lender be aware of this I believe the lender could cancel the mortgage?
Will the rental income be undeclared on tax return since on paper you will be living there?
A buy to let mortgage may have a slightly higher rate but since interest rates have just been cut to lowest ever in history (!) there should be some good rates. Interest only mortgage should free up cash for next deposit.
What about getting a 2 bed as your first property and having a lodger? This income would be tax free and you wouldn’t be breaking any terms and conditions.