hi tantalus-
well, if you want to leave, you should. we have a saying here "let go or be dragged" ...
since you live in the UK, she will have access to medical support without you.
i think if i was you, i would put together a plan of exit and execute it. due to her fragile emotional state, it might be best to not tell her or forewarn her.
if it was me, i would make my plan, exit, tell her as you walk out and also, call the police to say you are worried about her safety to herself. in the uk, they will come and take her to a doctor for evaluation.
as for the children, well, they aren't children at 18 and older, are they? i would carefully weigh telling them of your plans before you execute them. it depends if they can keep a confidence or not. if they can, they might want to exit themselves, as the burden of their mother's excesses will fall on their shoulders if they are last man standing.
for yourself, have you considered therapy? i find it very helpful in sorting out everything which has occurred with my xABF.
naive