IAD is my home airport (and work, given that's right next to the airport).

I've never been able to get into the BA lounge on anything but a BA flight. Their rationale is that the capacity is just too limited. Consider the space, then think about an A380 full of status passengers.
That being said, my last F trip had only two tables occupied in the "First Dining" area. You would think that GGL passengers wouldn't overwhelm, but I'll bet that IAD-LHR is pretty status heavy.