1. The house is your father's.
2. If he is the sole owner he can gift it to you if that is what he wants to do. If his wife is a joint owner, she would have to agree as well.
3. If he hasn't done so, he must make a will. So must his wife.
4. If he dies before his wife his IHT free amount is £325,000. If he leaves everything to his wife no IHT is payable when he dies unless he has made gifts in the 7 years before his death which exceed £325,000. If he has made no such gifts when his wife dies the IHT free amount is her £325,000 plus his = £650,000. If the estate is then worth less than £650,000 no IHT is payable. If it is worth more, then IHT is payable at 40% on the amount above £650,000 (not, as your post seems to say, on the full amount).