The Advance Directive is legally binding in most cases and certainly with regard to DNR and refusal of treatment.
You’re absolutely correct that no one can be compelled to have a Power of Attorney although it does make sense to have one just in case something happens which is not life-threatening but leave you without the capacity to be able to manage your affairs.
If it’s a physical incapacity, you could always do a power of attorney at the time but if it is a mental incapacity, then someone else could apply for deputy ship over your affairs to look after them. So whilst it makes sense to have a power of attorney, you don’t have to have one.
Voluntary euthanasia and assisted dying is still illegal. Any provisions in that respect would currently not be enforceable. By all means make your wishes known.
Carrying a document of some description perhaps in your wallet and some kind of reference in your mobile phone under ICE as a contact (In Case of Emergency) may help as would one of these USB sticks which people carry round their neck containing information such as this.
With regard to any post-mortem, the body should be reassembled although there have been cases where organs have been illegally removed and kept for research purposes. However it is illegal.
Once the body is released, it would be dealt with in the usual way by the undertaker, cremation or interment.
Wishes in a will as to cremation or interment are legally binding but it’s unlikely that if the executors didn’t follow those wishes, anyone else would want to take legal proceedings to make them. However, it could be done.
You are able to refuse any treatment including psychiatric treatment as you have described.
The statement is made by a commercial organisation who have a vested interest in getting people to use their service. However, if someone has died, how would they know that you have signed up to this service?
You would be surprised how quickly after a death beneficiaries go searching for a will. We had one family come into our office asking for a will (they had no death certificate so couldn’t have it anyway) and when we asked when their parents had died, the answer was “15 minutes ago!”.
The secret is to make sure that everyone around you knows what your wishes are regardless of what’s in the will.
With regard to the will company you propose to use, I don’t know whether they are good bad or what. I can tell you that there are lots of instances where wills are lost because the company that is holding them has gone bust. If a solicitor closes down, there is always another firm that takes over the documents and records. Most solicitors will not charge for storing a will but may charge for retrieval.