Diabetes is an insidious and dangerous disease. If your glucose level is near the max allowable, it may be just a matter of time before you cross that line. The impact of untreated diabetes is not minor. You can expect cardiovascular changes, greater risk of heart attack, progressive vision loss, kidney failure, impotence, and in extreme cases gangrene and limb loss.
You have to decide what's more important to you: passing the next medical, or a long and healthy life?
If you haven't crossed the line yet, I urge you to make lifestyle changes immediately. When I was diagnosed a couple of years ago I immediately lost 20 lbs and increased my exercise levels. I managed to put off going on medication for about a year. I hope to lose another 20 lbs this year. So far I only need metformin.
I don't know about the UK but in Canada you can fly with type II diabetes if you only require oral meds, and if cardiovascular, renal and eye function is still unaffected or within reasonable limits. When insulin-dependent, there are very strict conditions under which you can fly with either a recreational permit (class IV medical) or with a class I medical restricted to as/with co-pilot; but the criteria are very strict.
I was grounded for 3 months after my last medical and had to provide a thick packet of medical reports (blood glucose levels over the past three months, exercise electrocardiogram, echocardiogram, doctor's report, 24 blood pressure monitoring). I'm happy to say I passed due to my excellent cardio fitness (3500 km a year of cycling, 500 km of walking/hiking in the mountains), and have my medical back with a 12-month restriction (for a PPL it's normally 2 years at my age, 55).
Bottom line if you only worry about your fasting glucose level for the next medical, and then return to your normal lifestyle and diet, the disease will catch up to you sooner or later, and then it may be too late, enough damage may be done to preclude passing a medical again especially if there's cardiovascular, renal and eye damage.