It is my understanding that BA can and does limit non-BA passengers to their lounges when they EXPECT to be busy as well as when they actually are. Personally, if I was paying for First or even Club on BA (or was a commercial client) and couldn't get into a Club prior to departure because it was topped out with non-BA flyers, I'd feel a bit cheated. This is likely much more prevalent at the relatively (to JFK,LHR) smaller clubs, like PHL, which is quite nice.