If by virtue of the Mental Capacity Act a person is considered to be unable to give or withhold informed consent, how does that leave someone who wants to issue an advance treatment directive ?
I am not sure I can answer your question. My knowledge of the Mental Capacity Act is a bit old.
However, it certainly was the case that mental capacity had to be assessed strictly in relation to each item that the person needed to deal with - so while a person may have been incapable of making a decision or handling one thing, that incapacity was not be generalised to everything.
Not sure that is of any help for you.
Have you heard of Independent Mental Capacity Advocates?