Try the cabbage soup diet! It's amazing for losing weight fast. You can lose over half a stone in a week if you stick to it properly - it should't be used for long periods though as can be dangerous. Works on a seven day period and you have to eat different foods on different days. First day fruit, second day veg etc, that sort of thing. You can eat as much of the cabbage soup as you want as the more you eat the more you lose - you use more calories chewing it than are actually in it!
Follow this link for info, recipe and 7 day plan
http://www.cabbage-soup-diet.com/
Good luck, I hope your date goes well!