Ginkate30, no allowances are made for exercise. Yes, 800 is very low so the advice is not to exercise or to take in easier in the first couple of weeks. Your body will soon get used to the low calories and you will be able to exercise. The problem with deducting exercise is people seem to think it equates to weight loss. So they say, I have done X amount of exercise so I can eat X extra calories. Unfortunately the calories in and calories out theory no longer holds true. For example you would have to walk 35 miles to lose one pound of weight. You would have to do around 3 hours of exercise for one chocolate digestive. Not suggesting you are wanting to tuck into biscuits but just illustrating the wrong thinking on this.
I havent checked your profile so dont know if you are diabetic. If you are, take extra care about exercise or be ready to have a small carb free snack if needed.