I do not think they are entitled to use the Fast Track, but I have never seen anyone being turned away from Fast Track at LHR T5.

I honestly cannot believe that everyone who presents themselves at Fast Track entrance is entitled to use it, so I suspect they are basing the Fast Track usage on self-declaration, i.e. through the act of going to the Fast Track entrance.

In fact my husband had gone through Fast Track when flying Y as a Bronze as my guest but the BP checker did not know that (I was well ahead of him as he was messing with his coat pocket so the BP checker would not have noticed him to be with me), and he got through without being questioned every time. I personally find it bad. They should have stopped him to question him about the entitlement (I was standing behind the BP checkers in case he was questioned, of course, but they didn't know).

I usually find that non-Fast Track is faster anyway, though.

Fast Track boarding, well people don't seem to get rejected from the fast track line very often, except at AMS!