I think advance consent should be allowed for conditions where the progress of the condition takes away the legal ability to consent, just as it is for transplant-accepting etc.

There was a couple locally here recently who killed themselves in their back garden, because they were going to have to go into a care home, and living in a care home they wouldn't be able to choose when their condition was unbearable to them any more - but they also couldn't bear the idea of their children having to care for them as their condition got worse and worse even if they (the elderly parents) were so far beyond meaningful cognitive function that they didn't care any more. The woman here with ME who has spent a lot of time in court over it has a similar argument - she doesn't want to die while she still has enough control over her body and mind to obtain means and carry it out, but as the law stands she has to do it then or put up with the risk of living on past the point at which she finds things bearable, because even if her partner could bear to do it she doesn't want him arrested for murder.

On the other other hand, I have also read about a woman who promised her mother to help her die if she got severe Alzheimers, but hasn't done it because her mother is absent but perfectly happy, even if she's not the same personality any more.