WHBM, good point and one I meant to include because a fair proportion of the Premium pax were going onto European destinations especially Zurich, Munich Frankfurt Paris and Stockholm , the original move to LGW and the inconvenience of travelling to LHR for a 1 hr 15 flight meant a lot of them moved to a JFK connection so I think BA created the current terminal to transit ratio.
All thee with 52 thousand people is quite impressive but as i said there is a lot of premium including a lot of First as well