This is a difficult one, I read the article and most, if not all, of the comments are unfortunately people who have had bad experiences when they have disclosed. I have done this myself at work, but only when it got to a stage where it was affecting my work and I couldn't hide it - I had to disclose whilst I was already in the post, as I wasn't coping and didn't have any choice. I definitely would not have disclosed otherwise. It doesn't really matter what the policies and procedures say where you work - including Local Authorities. My experience of Unions was terrible. Its easy and I think a bit naive - to say - always be completely honest - but if you are on the breadline and have to earn money to survive with a roof over your head etc, I would definitely not put my trust in any organisation or management structure to support me by telling people about it if I didn't have to. If you have got nothing to lose, and have other financial means of supporting yourself - then well yes, go ahead, you won't lose your livelihood or home - but most people don't have that luxury. Its unfortunate, but the world is a harsh place and whilst you might get lucky and get someone supportive, you might not. Its too much of a risk to take if you don't have to tell. If you do have to tell, be prepared for a fight, to prove that you are actually ill, and to try and get what you need to try and remain in work - because from my experience that is what you have to do - and that really isn't what you need, when you are not well anyway.
For a new job, you don't have to tell anyone about any health conditions until you have been offered/accepted the post, so I would do so at that time, then leave it up to them if they want to withdraw an offer. I would definitely not go down any tribunal etc route, as its expensive and stressful, unlikely to win - and definitely not a good idea for someone who has mental health issues, far too much pressure.
I really don't think it should be up to people at their most vulnerable to fight a system for other people..I think that's a bit unfair and puts too much pressure on people who are unwell, whilst they are trying to survive with little/no support in the world of work.