There aren't many choices at IAD and they are all just ok. BA lounge does double as a PP lounge, but I think PP users have a time limitation for entry. I remember being turned away once using my PP status, and was told PP users can't access it after a certain hours. I have forgotten when.
I don't see any other lounge that Cathay can use except the BA lounge. Cathay personnel (probably just hiring BA personal with Cathay uniforms) will take over at Cathay flight times.