Operating another A380 via Hong Kong would sure let a cat loose among the CX pigeons in the immediate and near future!
With 6 flights a day to LHR, plus directs to LGW, MAN and around 10 other European cities I doubt CX would lose much sleep over over a single daily QF A380
The timings of the QF MEL-HKG A380 mean a six hour layover in SIN until QF1 departs for LHR. Better off taking the later QF A330 MEL-HKG and have just a 90 minute connection in SIN to QF1