I have a few points to make, and don't think you are being unreasonable.
To all the people who have commented on legal matters, HMRC, etc etc.... FFS we don't know where in the world this house is, so STFU. I'm going to address this more strategically.
The decision on this rests on a balance between morality and finance.
Addressing the finance aspects first.
Can you calculate how much money you will lose by selling the house now, rather than after your father has died? The answer to this is no, because none of us know how property will perform in the coming years. But you do know that you may be liable for a tax similar to CGT if you sell now. I have no idea whether you would be liable for this after his death. So you can do some calculations on THIS bit of the sum. When you are trying to address the impossible (property values in x years) bit, don't necessarily assume that property will continue to go up. Property values in London are definitely falling ATM and I expect this is happening/will happen in other areas and countries as well. Just my take on thsi bit.
Now the morality bit.
Your father wants to be more comfortable in a smaller flat. How much are you prepared to pay to allow him to be more comfortable? Does this amount include an allowance to allow him to have more income? To those who think the OP is a bitch - would you want the OP to give him money if it turned out the reason he is so poor is because he is a gambling addict? Is it actually a good idea to increase his income, irrespective of whether the OP wants, or is prepared, to do this?
And a new idea bit.
The main reason that has been given for him wanting to move is heating and maintenance costs. Is there any way the OP could have the property developed so that father lives in a SC flat within the property, the rest is either rented out or just left empty (yes, it will need maintenance, but depending on some of the questions raised above it might save IHT type costs).
Gets popcorn, sits back and watches the fun. Good luck, OP, with your decision.