I've been wearing mostly UK's for five years now, and they don't really blow up as much as you think. The outer apron will flap open a bit and show some thigh, but the under apron stays pretty much in place.

With the snaps placed so, the parts you REALLY need covered can't really be seen just by the front apron blowing open. If it really makes you feel uncomfortable you might try adding some weight (like for drapes) into the apron hem. Avoid pinning the two aprons together - that's a recipe for tearing the fabric.