I flew LAS-LAX-LHR last week. It was AA booked itinerary but BA metal for the LAX-LHR leg. VeriFLY was accepted at the American check in desk and no further in person documents were checked at all, nor at LAX on the connection.

I flew again from LAS today (not to U.K. this time) and noticed signs up all over Aa check-in advising to use VeriFLY if you can.