Yes, they can do. In a lot of cases it's not to get at you, but to cover themselves in case anything happens to you while you're at work.
The result of an Occupational Health assessment is supposed to tell them what alterations/allowances they need to make in your working environment to enable you to do your job as well as possible.
In my last job I was referred because the job involved a lot of outdoor work and walking over moorland etc. At the time it was becoming obvious that I had some mobility problems, though I had not yet been diagnosed with lumbar spinal stenosis. I could still walk (say) 5 miles, but I had to do it at my own pace so I wasn't as fast as a younger, fitter person would have been.
I was convinced they were doing it to get rid of me, but in fact they wanted assurances that I wasn't (eg) going to have a heart attack while out alone! I had all the appropriate H&S equipment, and I was allowed to set my own work programme and timetable. A colleague who covered the adjacent area would cover some of the things I couldn't do (a 20 mile walk over part of a national footpath, for instance) while I would do a few of her smaller jobs to make up for her time.
You can make an Occupational Health Assessment work for you. They are independent, even though your employer pays for it, and the aim is to keep you in work. I would advise you to be completely honest during the assessment, just as you have been in your post above.
Your diabetes is under control. There's a valid reason for you 600-cal diet, I assume? Your dyslexia has only recently been diagnosed - are you having help wih that? You could ask, at the assessment, if there is any help/training available that might help you in the work place? though it seems you are already holding down your job okay. Maybe ask your GP or whoever for a letter giving confirmation of your date of diagnosis, if you don't have something similar already.
One tip - I always get our practice nurse, who doesn't freak me out at all, to take my blood pressure before I go in for anything like this. They'll take yours at the assessment, and it helps if you can blame it on 'white coat syndrome' and give them an indication of what's 'normal' for you! :lol:
Try not to worry about it - and good luck!
Viv 8)