Not an expert so just initial thoughts:
If you buy it will be classed as second home and stamp duty will be additional 3% but maybe less than IHT
If you buy it, then your Dad buys it off you and transfer ownership, it will incur 2 lots of stamp duty and legal costs
If you buy it and your Dad gives you the money, but pays no rent and you are still the owner I'd have thought there could be IHT considerations, and if he needs care in the future council can still see this as an avoidance approach
Are the houses moving that fast that he would miss out being part of a normal chain?
Anyway, nice thought I hope you can work something out.