I work in critical care. We often reach a point where treatment is futile and treatment is withdrawn. The various support systems and drugs we have running are gradually withdrawn with the knowledge that the patient will subsequently die following the removal of this treatment. This is done because treatment is futile, the patient is going to die and all we would do by continuing treatment is prolong suffering. So I suppose, in a way, we already do this within the confines of the law, however it's a cessation of treatment rather than actively administering something to end someone's life.
Palliative care in this country isn't the best it could be. It started off as something that was more of a speciality confined to cancer specialists, but is starting to broaden and patients have better access to services at the end of life. It's something that we're going to need to get a lot better at with an ageing population, but some of the best guidance and pathways we used to use have been removed following misuse by some practitioners and a hysterical campaign by the tabloid press. People should be able to expect good quality end of life care, with pain and anxiety well managed, with any distressing symptoms of their illness completely under control. I think if we were better at this, many people might not see as AS or euthanasia as their only option to avoid suffering.
On the flip side of this, there are some horrific diseases where your body fails yet your mind keeps going, such as motor neurone disease or locked in syndrome as people have already mentioned in this thread. I have a friend who lost a parent to MND, and I can completely understand why anyone would want to choose to die over the suffering that a disease like that can cause.
Over the Easter break