At the risk of being irrelevant and off topic, I would point out to those on this side of the Pond that the UK is governed by an "unwritten constitution," a body of traditions rather than codified regulations (although I believe this is not as true today as it was decades ago). The winner of the most seats in a parliamentary election goes to the queen to announce formation of a government. The queen really has no choice of whether to agree or not, but that's understood, not as a matter of law.
The Parliament is the supreme authority; there are no co-equal branches (although Matt relates that the judicial system is more independent than previously). The executive is a function of the Parliament, not a separate branch. But the bottom line is that what Parliament says is, essentially, the law.
An old example is the Asquith government's threat to remove the House of Lords' veto power or, in the alternative, flood the House of Lords with Liberal members should the Lords decline to approve the Commons' budget. He won, essentially altering the British constitution.
As to the original question: It is a crushing responsibility to decide whether someone's life should be prolonged when there is little hope of recovery. Been there, done that. Knowing what I know now, I would have argued against some of the treatments that were offered. But I can never escape the decisions I made.