The BMI thing has long been a vexed issue, as an unfit skinny person can breeze through where a well-muscled footy player can struggle. I know of one bloke in particular who played schoolboy rugby union for Australia who had to do all kinds of silly things, like dehydrate himself, to jump through the hoops.
One thing to keep in mind if you're trying to get your weight down is to avoid faddy diets etc, because you might find it plays havoc with your blood cholesterol etc. A balanced low kilojoule approach and aerobic fat burning exercise would probably be the go.
You may just have to pretend to be a supermodel for a while and eat a lettuce leaf a day! Good luck.