Dropped out of PhD - should I seek acknowledgement of disability discrimination?
First of all, if you're able to get legal advice, you should do that. Find someone who specializes in higher ed law, which might be a project by itself.

I'm not a lawyer and I'm not well-acquainted with UK disability law, but it does sound to me that what your department did was illegal discrimination and harassment. From the details you've provided, it also sounds like it would be impossible to prove and no lawyer will touch it on a contingency basis because there's no clear financial loss. It's also extremely unlikely that the university itself is likely to find in your favor if you go through a university grievance process. So the bad news is that there's probably nothing you can do to clear this from your record.

It's probably not much consolation, but I've heard dozens of stories about graduate programs engaging in exactly this kind of "soft" discrimination, especially against students with more amorphous disabilities such as chronic illnesses. The mentality seems to be that a student with this kind of disability is unfit and a waste of resources. This reasoning is explicitly illegal, but graduate programs are granted so much latitude in evaluating students that they get away with it.