1. CX is rolling out a new J class that is very much like the new US Envoy (quite similar to BA F). Depending on when you are traveling, it may be on some HKG-LHR flights.

2. BA 744 J is very good if you can get good seats - 64AK or 62AK 14AK on Hi J. These are all very private and the ones on the UD (6x) have an extra shelf and storage.

3. From your itinerary I would imagine it is a J RTW fare. If so, that is capacity controlled so you may not end up having as much flexibility as you think. I believe QF still has a 1x a day HKG-LHR flight but it is a 744 and would have ski slopes, so CX or BA would be your flat bed options.