Me, my sister and my Mum have a power of attorney and my Dad has an end of life plan. The last time he was about to die - which amazingly was 2.5 years ago now and he's hung on since - the hospital wired him up to the ABs by the time I got there. I did try to say that he didn't have a great quality of life (he didn't, and he's got a rare degenerative disease that will kill him slowly and even then was immobile and incontinent, although could hold a conversation for a short time) but my Mum shouted over me that he had a great quality of life. The consultant came back to ask us who had POA but by that point he'd pulled back from the brink.
I then managed to get a copy of his "end of life plan" from my mum and it was awful. My Dad wasn't daft and I don't know what he was thinking. It was a template with a series of tick boxes about in what circumstances he'd like to be kept alive then some quite vague and contradictory statements about his religious beliefs.
He'd also previously told my sister he wanted a DNR and had told us all previously, for years, that he didn't want to be kept alive in a care home.
My mum is, however, absolutely determined to keep him going. I don't really speak to her anymore (for this and other reasons), but she claims not to know what medications he's actually on, and basically authorises and campaigns for any medical inventions she can get.
She even tried to get DF referred for cataract surgery and went around badmouthing the local optician when he refused to do a referral without writing it was on her request on it. Also she claims Dad communicates with her although he hasn't been able to speak to me or my sister for well over a year, and not made any sense since the episode with the hospital.
So no, no clear plan and even if there was one I think it would be quite difficult to somehow overrule her.