Taylorcat while I can say for certain that this is not a situation for which a codicil to his will would work (because that only activates after death) I have no idea what the current state of play is re a living will in Scotland. Has your father drawn up a welfare power of attorney?
I think the potential problem with any living will is that while any restictions contained with in it might be considered if there is time to do so, in the heat of the moment, in say an emergency, SOP is to make every effort to extend life without consideration for the consequences. Also, of course, it seems to be up to the treating doctor and their view on their duties and responsibilities as to whether the expressed wishes contained within the document have any validity.
Edited to add: as you can see from the links Hazel posted, while a living will is worthwhile to have, in most circumstances in simply provides guidance rather than being legally binding. So validity is not really the correct question, the correct question is how much credence will an individual doctor give to it. I absolutely believe that it is a worthwhile thing to do, but we all have to understand that it can be ignored.