Lots of people have been talking about "well, no matter how bad it is, you don't kill yourself if there isn't an underlying problem, and you can't blame cruelty for that."

I have just had this debate with my consultant! She reckons that people can't withhold consent to be treated after an overdose, because once they lose consciousness they no longer have capacity to make that decision. I think that if they've made a statement to you that they don't want to be resuscitated, you have to abide by that. Suicide does not necessarily equal mental impairment, however much we as doctors might like to claim it does so we can stop people dying. Having to let someone die is awful, but I think if that really is their decision we really don't have any other option.
In practice, it won't be my decision for several years, and even then I would ring the Medical Protection Society and go 'aaargh' at them for legal/ethical guidance. But the conversation was good to have! It was one of those good debates where people listen to the other person's side.