I say go for it! Losing a lot of weight and committing to regular exercise can have a dramatic effect on blood sugar levels. Losing weight and reducing carbs is a very effective way of reducing insulin resistance, and is the key to dropping blood sugar levels. So your GP is right when he says you may reverse your diabetes if you lose weight.
I lost 3.5 stones in six months and dropped my HBA1c from 83 at diagnosis to 39, and improved my cholesterol. My HBA1c has fallen further since then to 36. I don't take any meds for my diabetes. I've kept up my exercise regime and continue to follow a lowish carb, moderate fat way of eating.
MY GP didn't think it was possible, was positively discouraging - so at least you have that advantage over me - boy, was he gobsmacked!
Set yourself reasonable weight loss goals ( I recorded my weight loss goals in a spread sheet, started a food diary and charted the drops in blood sugar levels. I could see that as the weight came off, my blood sugar levels fell and this encouraged me to keep going - as it was obviously working! I walked and walked and walked and using a pedometer, steadily increased my exercise levels. Exercise became easier as the weight dropped off, and I began to enjoy it.
You've nothing to lose - except 5 stone - and everything to gain: great control of your diabetes, and a better healthier, fitter, slimmer and happier you!
Gobsmack your GP - but more importantly, take back control of your health.
Good luck. I wish you every success, and let us know of your progress.