Sorry that things may go south so soon Peter; so it's essential that you get the will sorted ASAP. If you die without having a will in place, then it needs to go to probate, and if you have to get a professional involved, then they often charge a fee, plus a percentage of the estate. A local solicitor should charge about £150 plus VAT for a basic will, but you can ring around and get a few quotes.
Have you had a chat with the kids yet; do they know how bad your health is, and do they expect to be able to blow their inheritance, (which I suspect you reckon they'll do) or make use of it.
There are no IHT implications, and you won't be giving them the property till the very end, so you need to decide whether you make a basic will, and leave them each a share of it, or you put it into a trust that's managed for them, so they can't access the asset until they are a little older.
At the end of it, they will either sink or swim, and if they decide to enjoy themselves, then they've got to deal with the consequences.