When I realised that the MEL-DXB leg of my LHR-AKL-LHR trip was unlikely to be upgradeable in a month of Sundays, I decided to cancel the booking and wear the GBP 200 fee.

I then booked LGW-BKK-LGW and have upgraded all flights except DXB-LGW, which I really don't care about.

So the moral of the story is if an upgrade seems most unlikely, just book another trip that looks better for upgrades (check on expertflyer).