Hello,
Losing weight by calorific loss isn't actually healthy or sustainable. Your body will simply go into survival mode. You are better to go on a healthy diet with plenty of protein, and instead doing a healthy amount of exercise with a little weight training.
As a size 10 you are actually considered slim, and under the UK national average. Unfortunately, there is still pressure (on, typically, young women) to constantly be trying to lose more weight. You are fine just as you are (I promise!), and an 800 calorie diet can actually be dangerous. Generally, the recommended daily calorie intake is 2,000 calories a day for women and people need an absolute minimum of 1,200 calories daily for normal bodily functioning. Only having 800 calories can lead to hair loss, gallstones, loss of periods, dizziness and nutrition deficiencies that can lead to a whole host of more problems.
Please take care fo yourself and don't be too harsh on your body - it works hard to keep you healthy everyday - and as a UK10 you are absolutely fine as you are!