Mate,
These BMI things are only a rough guide. For example Jonny Wilkinson the England Fly Half is about 1.75 tall and about 90 kgs, his BMI would work out at 29ish which is really obese.
If it helps you get through the initial application fudge the weight a bit. But having a BMI of 19 is ok I reckon.
Cheers
BB