A few years ago when I worked in a travel agent selling BA fares to China, we were always able to see availabilty for CA connections, but 99% of the time it was false availability. I don't think I booked a single BA+CA combo via the BA portal (Lime Managment?). I always ended up either booking CA the whole way, or BA international, then whatever Chinese airline that fitted the itinerary. Seems the false availabilty is still an issue.