Can I just clarify that in England and Wales, health care professionals are legally bound to comply with a living will (advance directive/advance decision to refuse treatment) as long as it is deemed valid and applicable to the situation.
For it to be valid you need to be 18 or over at the time of writing it, you need to have capacity at the time of writing it, and it should clearly state what specific treatments you are refusing (i.e. life sustaining treatment, intravenous fluids, intravenous nutrition etc) and in what circumstances you want it to apply (eg after a stroke, cardiac arrest, living with dementia).
In addition, if your advance direction refuses life sustaining treatment it needs to be in writing, include the phrase “even if my life at risk” or similar, and be signed by you and witnessed by someone who should also sign it.
For it to be applicable you must lack capacity and be in the situation the direction refers to and the treatments you have refused are the treatments that would be given. There must also be no doubt that you could have changed your mind since writing the direction.
If you appoint a lasting power of attorney they may be able to override your advance decision but you can choose whether they are allowed to make decisions about life sustaining treatment or not. If you don’t give permission for that they cannot override it.