I don't think committing suicide is illegal any more.

In the UK, the NHS is for everyone. Arsenic has no known value apart from being poisonous, delineating is slightly from tobacco and alcohol. As the country has to pay for one's health this gives the country some say in what one intakes.

One could argue that anyone deciding ot do such a tihng is a danger to themselves and hence can be sectioned under the Mental health Act.