I encountered a similar problem a couple of days ago. End result the same every time I attempted to book the seat. When checking availability it would be there on minute and then gone the next. I rang BAEC and the agent encountered the same issue several times. She ended up saying that it must be a problem with the link between BA and CX or that CX are playing about with availability and that I should try again in a few hours. This was with regards to a shed load of CX F availability that began to show for LHR-HKG the week before Christmas.

I ended up booking LHR-FRA and FRA-HKG as two separate tickets and saved a fortune in UK ADP!!!!