I only have experience on land based military ambulance matters, but can't really see any contradiction of the convention in what the UK do.
The baddies are not bothered about the convention it seems, and even if they were, could they really tell if the dustoff was on the way to or from an unseen casualty?
The Brit army used to (don't know if they still do) put foldable red crosses on their ambulances, but to be honest who in their right mind would want to go to the outside of a chinook and unfold a red cross to show they were not to be shot at once they had picked up a casualty
