You might also try getting an Asia Miles account and checking there for availability. It's bound to be more accurate. CX is famous for holding and not releasing seats it thinks that it might be able to sell itself.

To the extent that anecdotal evidence is worth anything, I fly LHR-HKG several times/year, nearly always on CX, and always on xONEx tix. In April, I couldn't get on the CX flight I wanted. There were seats available, but nothing in A class. When I got to LHR, I walked over to the CX ticket office and asked. They were happy to give me the last F seat for my xONEx ticket, an A class seat. (Of course, I had to shuttle back and forth to the AA ticket counter a couple of times since I had booked my itinerary with AA, but it worked out quite smoothly.)