As most of us will probably die in hospital, there is no real risk of any of us being buried alive.
I don't know what the procedure is in the UK now, but I can assure you that in Denmark the physician signing the death certificate has checked the deceased's pulse, listened to their heart and lungs through a stetoscope, touched them etc.
And if there is any doubt, the death certificate will remain usigned until a second opinion has been sought.
The undertaker too is used to seeing and handling the dead, and very unlikely to make a mistake.
I doubt that even the most inexperienced of us with no medical training would be in any real doubt. The dead become cold to touch very quickly - far colder than any living person or animal I have ever touched.
In hospital a doctor is no longer relying solely on the clinical signs of death that he or she can see or feel, but also has a range of sophisticated eletronic equipment at hand.
By law here, a watch is kept by a deceased person for a certain number of hours before the body can be prepared for burial.
By aquainting yourself with the procedure that the certifying physician is legally obliged to follow, I think you will find a good number of your fears are resolved.
Whether you choose cremation, burial or to leave your body to science is a personal choice that no-one really can help you with. My preference is burial, but as far as I am concerned the important thing here is to be certain that your spouse and children know what you want and are accepting of it.
We do all have to go sometime, and no, very few of us can honestly say we are philosophical about it - what hurts me, if I allow myself to think of it, is being the first to go, as leaving my husband to fend for himself - oh, dear no, I know how utterly miserable he would be without me - I shall miss him like h* if he goes first, but will be truly thankful that it was that way round.