I don't know the up-to-date employment regulations but I do know that, about 10 years ago, when I was suffering with diabetes related conditions that caused me to need repeated time off work my employer eventually asked me to attend a GP's appointment for a medical report (which they paid for). The employer then was able to end my employment, acting on the content of the report, and covering themselves by stating that it was a matter of health & safety.
I can't imagine that much has changed and I guess that disability rights have become more powerful recently. I would expect that any job change resulting in a loss of money for the employee would need to be mutually agreed between employer/employee, unless recommended by a company nurse or your GP. I think that you should get some advice from your local CAB as they are very good on employment law.
There are a lot of "fine points" regarding employment and illness, so I suggest that you document everything that happens (dates/times of incidents at work and sick days, who deals with you, who tells you things like "you have to take less lunchtime as a result of hypos" etc). All these things are evidential if you have to go to a tribunal or arbitration.