Just to add some thoughts from an employer's (and personal) point of view..
By the time things get to the state south869's describing, it's probably best to get out while you still can. If the employer/line manager has decided you're a problem, then they'll be looking for ways to manage you out. That can quickly become a personal thing if the manager's decided you're challenging their authority. Even though in this case, the manager is being a **** and opening up the company to liability issues.
Problem is that although the discrimination Act(s) provide protection from discrimination on grounds of health or disability, the result can be a lose/lose situation. So to 'win' you may need to go to tribunal, roll the dice with the panel and you might win the right to keep your job, or some compensation. But it can be incredibly stressful, expensive and time consuming, and if you win, you may find the employer's still going to try to manage you out.. Which leads to more stress, and sounds like depression is already setting in. That's not good or healthy. Sadly though, it's business, albeit bad business.
As for things like health questions at interviews.. That's part of the minefield. In my experience (and preference) it's best to do those in 2 stages. So first interview would be about whether the person can do the job, and how good a fit they seem to be for the role. If a candidate passes that interview, they're handed over to HR to ask the awkward questions. That's partly due to the risks in the discrimination act, ie dealing with any suggestion of discriminating against a candidate, and also a good HR person can better understand any adjustments that may be needed if they're hired. There's also data protection and privacy issues to consider.
But all I'm interested in is whether the candidate can do the job, or not. From my experience, diabetics have never been an issue as it's pretty much self-managed by the individual. Adjustments have also been minor, ie a bit of awareness.. Which I still think is a key issue, ie other staff may be nervous of seeing insulin in a fridge, but that's easily manageable. There's a wide range of first aid pouches to keep medical stuff in, and staff can be warned not to touch someone's snacks.
And being recently diagnosed myself, probably also opportunities to raise staff awareness. That may need some sensitivity, but might also encourage other staff to get checked. I suspect I've been diabetic for quite a while without realising it.