If my father 'sold' me his house for very little....

[LittleMissRayofHope]

If my dad decided to sell me his house for a small sum, how legal is this?

His house is worth around £350-400k. I have small savings. Less then 20k.
If he were to sell it to me for £100, so I would still have to obtain a mortgage.
How legal is that?

On the surface it seems completely fine but will the 'lost value' be viewed as gifting and therefore require a taxation?

As you are a connected party it is likely that you will have to pay stamp duty on fair market value

As it is not his primary residence there is likely to be capital gains tax and this is not just an inheritance tax question

As the property is rented out it is seen as an investment. Your father will be liable to capital gains tax on the market value less the market value when the property ceased to be his main residence. I think capital gains tax can be up to 28% of profit. He will have a personal allowance of £11,000 that he can offset against any profit, but he would be liable for capital gains tax on any profit over his personal allowance

Another thing to be careful of is Pre Owned Assets Tax (POAT). He can't be seen to be enjoying the asset he has gifted to you. So, he can't come to live with you and shouldn't stay at your home regularly etc.