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Unfortunately no A380 availability HKG-SYD on the days I can get out of Europe. The outbound is BA F SYD-SIN-LHR on 777.Something that has not been mentioned is kinda like an F reversal of your outbound.
ie BA A380 F LHR-HKG (1420 arrival) and QF A380 F HKG-SYD (1945 departure) - that is, if you get it ex-HKG on Thurs/Fri/Sat/Sun? Not sure if your travel date has any flexibility?
You could originate in a European hub and connect in LHR to avoid the APD and get to try the BA tasting menu.
With your outbound, do you mean QF J to SIN, then BA F to LHR?
Not sure if it would work out but you may consider BA 777 F SYD-SIN??