This probably doesn't answer anything, but I learned it in law school.
After death the body doesn't belong to the decedent anymore and can't be bequeathed or its disposition restricted unless local law says otherwise.
Which it does in certain organ donor laws. But what if my Florida driver license option (or whatever) is not honoured in the UK?
BTW this is an argument (in the USA) for execution by firing squad: the condemned's organs, with his/her consent, can be recovered for transplantation.