Ok, apologies for my earlier erroneous post in that case!
So there's really no point to a BVA unless your 457 is a few months away from expiring.
Would be nice if they clarified this on the issuing note. They just say if your 457 "ceases", BVA will kick in but it's only true if it expires at the end of the four year spell rather than 90 days after you leave your sponsor.