I was a regional entrant to WOTY last year on behalf of my group. As I understood it to get into the regional finals you had to have lost over 3st. There was no mention of BMI and I wouldn't have a clue what mine was at the time, although I could probably work it out.
I was chucked in at the deep end somewhat because I was only told about it the Monday before the regionals and I had no idea what to expect, I'm not a confident public speaker and at the regionals you do have to speak to a group and then if you make the final of the regional final (which I did) you have to address everyone, and that I found very hard.
Still, it's a really nice day out if you do get invited to the regionals, just go prepared!!!