Unless the law has changed, I do not believe it has any legal effect.
You can expressly decline medical intervention, but I do not believe you can say that you (would) want it.
Well obviously, you can say whatever you like, but my understanding is that the purpose is to prevent interventions that you wouldn’t want, but it cannot bind anyone to interventions that you would.
Tina
Tina, I am sure you are right - you have a much better grasp than I do of the legal nitty-gritty. I was thinking more in non-legal terms of it just being really helpful for doctors dealing with an insensible person to have an idea of how, in broad terms, that person might want them to play it, were she in any position ot comment on the matter.
Alison
You might be interested in reading “On mortality” by Atul Gawande, an American doctor. This is an excellent book on (as he puts it) illness, medicine and what matters in the end. He makes clear his opposition to assisted dying and has lots of interesting things to say about the role of doctors for people with terminal or degenerative diseases or just the effects of ageing.