Because CX does not have any offer F currently from any port in Australia to HKG. Depending when your travel is you could go J to HKG, and then F onto LHR.

CX did not offer F into SYD for most of last year, and I am not aware of plans for CX to reintroduce it on that sector.

CX did offer F to MEL on some flights, and I flew HKG - MEL in CX F as recently as January. Currently J is the highest cabin on this route. However I noticed when doing some planning for travel in December 2009 that CX F reappeared as on option on MEL-HKG.