Blood glucose levels vary with what you've been eating. (My mother is a diet-controlled diabetic, so I've seen this at close second hand.) Cut right down on extra starches and sugars between now and your medical, and drink plenty of water, and you may find you don't have a problem after all.
You are very unlikely to get rejected on this issue in the long term, judging by what has happened to other forum members. It's more likely, depending on the particular MA, be given time to follow some treatment and/or an exercise/diet regime and have to report back, or you may be asked for the opinions of more doctors. Or they may just say, 'Oh, okay.'