And unless you knew Thatcher personally, you cannot know which was the case for her. So I am going to assume she was sane even after she knew of her condition until I see factual evidence to the contrary.
Nevertheless, as I typed, it should be up to the state to have to prove you are not sane, not the other way around.
And no, it would not be easy. But that is fine also.
It should always be difficult for the state to take basic rights and freedoms away (which committing suicide should be)...not easy.
Sane until proven insane.