OP I used to have anorexia, so lots of personal experience getting irritated with weight loss plateaus on very low calories! First off, it takes a little over a 3000 calorie deficit to lose a pound of fat. To lose a stone of fat in two weeks while eating 800 calories a day would mean you had a total daily energy expenditure of about 4000 calories - very unlikely unless you're a professional athlete! A lot of that initial weight loss will have been water weight rather than fat, and it's completely normal for that to cause an initial rapid drop followed by a slowing or plateau. There's absolutely no way you're sustaining yourself on 800 calories a day unless you're in a coma (ignore all the 'starvation mode' nonsense, there's no evidence for that at all), so you will still be losing fat, it's just going to be hidden when measuring your total weight because of other factors.
Obvious one first - are you weighing yourself first thing in the morning, naked or wearing very few clothes, preferably after going to the toilet? Sounds stupid but thick clothes can add up to a pound, and you'll gain a few pounds over the day from the weight of undigested food and water.
Water weight is a big one when you first start losing. Counterintuituvely, the way to get rid of it faster is to drink more water - this helps flush out excess salts, which are what causes your body to retain water in the first place. You'll also need to drink more than the average person anyway because a lot of our water intake comes from food, so eating less means you need to get it through some other means. If you get periods they can also cause water retention and you'll probably see a big drop when your period finishes.
Are you 'eating back' the calories you burn during exercise on top of your 800/day? Fitbits and MyFitnessPal tend to overestimate calories burned during exercise by a significant amount. If you are then that's possibly something to bring up with your doctors as they can advise you on how best to manage that.
Finally, a VLCD usually gives you constipation and that can contribute to your total weight. I've never used laxatives because they can be addictive when you have an ED, but again if this is a problem it's something to bring up with a medical professional. Some more natural remedies that worked for me were drinking a tablespoon of coconut oil melted in instant coffee daily (yes it's disgusting, but you get used to it!) and crouching over the toilet bowl instead of sitting on it (ditto).
On that calorie intake most women could expect to lose about 3-4 pounds a week. As a PP said, you've averaged almost exactly that over the last month, so the diet is probably working as expected even if it's not causing your total weight to drop in a linear progression. You also have amazing willpower to stick to such a strict regime, and I'm seriously impressed! I hope you manage to achieve all your medical goals successfully.