A BA agent refused access to a OWE (who happened to be a GCH) who is flying oneworld into a very small lounge of which she knew that it was going to be over-subscribed based on the state of the bookings on that day. The BA agent also knew that its own passengers do not have an alternative (as there is no other OW lounge in the terminal) while the said passenger already had access to another perfectly fine lounge based on his own booking. So yes, in my view, that is both understandable and acceptable.

If the BA lounge was large enough to accommodate CX passengers, you can be certain that CX would choose to send its passengers there rather than in the SK lounge. They have looked at that less intuitive solution for a reason - namely that the BA lounges already get very full with the BA passengers alone even on 'average' days, let alone on very heavy business traffic days.