Sugar Kane
Firstly, your health/not being exposed to Covid has to be the priority.
Second, if you need an Occupational Health interview before returning to your normal workplace (and it sounds like someone from your organisation has recognised that you do) then you can legitimately refuse to return to your normal workplace until this is carried out.
Once you have the OH interview, you can present any concerns you have, including any evidence from medical professionals familiar with your particular situation.
Your employer has to ensure that it's safe for you, specifically, with your conditions, to return to work.
If they can't do that, and it sounds from what you're saying that they can't, then they can't insist on you returning to work.
You should probably get advice from someone who knows what they're talking about (like a union if you're in one) rather than some random off the Internet, but if your condition wouldn't prevent you from working in normal, non Covid circumstances, then I don't think they can force you to leave your job by taking medical retirement against your will.
But whatever the eventual outcome, it sounds like it's something you need proper advice and support with now so that the worry and uncertainty don't become overwhelming.