You definitely need the union on side.
As an employer I can tell you that it is illegal to discriminate against someone with a disability, but you aren't required to continue to employ someone incapable of doing their job.
A company the size of Boots should be looking at what work they can find you that is appropriate to your abilities. But only if the situation is likely to be permanent. They are entitled to ask for a report from your doctor about your prognosis and capabilities, but they cannot criticise your doctor's choice of treatment. But if you are self medicating you need to think carefully about what you admit. I think in your situation I might admit to supplements but probably not NDT sourced on the Internet.
Your new manager must not harass you, or attempt to force you into inappropriate activities. In my opinion that includes disciplinary action and if the union don't come back pronto you should contact occupational health and set him straight pdq. They will know that such action can lead to an employment tribunal they cannot win!
If the union aren't as helpful as they should be the CAB has specialist advisors, and even lawyers. It would be perfectly reasonable to ask for a stay to wait for them to be available in my opinion.
ACAS are very good on the law, but when I had an employee with cancer their first reaction was to tell me how I could sack her! All I wanted was to report in regularly because she was working unsupervised and I needed to know she was safe and hadn't blacked out! They did finally agree that was reasonable but I haven't consulted them since!
Overall I would suggest that you try to keep the confrontation down, and ask for support while you sort yourself out. You should know your rights, but employment, even when your manager is a prat, is a partnership, not a war.