BA/QF have a joint lounge that is operated under their joint services agreement for flights between Australia & UK via Asia. Though neither airline operate a through service via HKG pre SARS you could fly from LHR and connect in HKG to PER, BNE, SYD, MEL and not too long before that CNS. The connection was quite good, only a couple of hours and the route flown was actually the shortest time in the air for Australian East coast destinations.

Pre SARS BA operated 2xdaily 747 and QF operated 2xdaily services to SYD & PER and daily 767 (at least) service to the other Australian destinations (except CNS).

So with this, around 8 departures a day between them the economics made sense to go with their own joint lounge rather than pay CX to lounge their pax. The joint lounge opened pre OW iirc.

Also considering BA and CX compete on this highly profitable and volumous route it makes sense for BA and CX to differentiate their products at either end.