Unfortunately, cancellations due to a diabetes diagnosis is not unheard of. There are rules (not guidelines) governing blood sugar levels where elective surgery is concerned. If the surgery is an emergency and life-saving, then that's different.
If the HbA1c is above a certain level (something like 8.5%, although I haven't checked that figure), the surgeon must'n go ahead, until the numbers are lower. So,
@DavidGrahamJones , diabetes is indeed a valid and appropriate reason to cancel elective surgery.
The rationale is that those with poorly controlled diabetes are more at risk of having poorer outcomes.
There is a national piece of work about to start, focusing on improving the surgical outcomes for poorly controlled diabetics, focusing initially on surgeries for cardiovascular conditions and joint replacements. I am on the Steering Board for that study.
It's obviously very unfortunate that your diagnosis came at the time of your proposed surgery Debzz, but the surgeon's hands were likely tied, in terms of deferring it. The best you can do now is to work hard to pull things back into shape as quickly as you can.
As far as your capability meeting is concerned, is this happening because you are no longer able to carry out part or parts of your job? I would happily comment a bit further once I know why the capability review is taking place.