Nigel, airlines most definitely do accept different passports for inward and outward flights, and this is irrelevant anyway as once you get off the plane you are free to stand in any queue. My children are all dual Australian/British nationals, its actually illegal for them to try to enter Australia on their British passports (I believe the USA has the same rule). But entering Britain they use their British passports. So they aren't in the wrong queue in either direction.