There are many diabetics who have been granted visas without difficulty (here is an old thread in illustration, and the official attitude is still the same
https://www.enz.org/forum/showthread.php?t=11353) - as I said above, it's a matter of the MA being assured that you and your doctor(s) are aware of your condition, and that there is a course of treatment in place to control it. If your doctor writes a letter to describe tests, diagnosis, the treatment and advice so far, and the outcomes so far (and your lifestyle changes and weight loss will be a large point in your favour), this will help the medical referral process go through as smoothly as possible when your case comes to be processed.