Does QF/BA not do what NZ does and have a crew based at the furthest outpost? How it works for NZ is LHR based crew do LHR-HKG-LHR and LHR-LAX-LHR, and NZ based crew do AKL-HKG-AKL, AKL-LAX-AKL (and the other routes too). In this set up no crew do the long AKL-LAX-LHR-LAX-AKL roundtrip (at least while working as crew).

I am not sure if BA has such a set up. I have met some SIN-based crew members on SIN flights but they only have a limited presence (ie the flight wouldn't be crewed entirely by the SIN base) - there were usually 2 on every flight, who'd do the Chinese announcements and presumably assist Chinese speaking pax. I was always under the impression that virtually everyone speaks English in SIN though???

QF has a LHR base which operates LHR-Asia. The Australia-based crew members operate the Asia-Australia bits of the LHR routes and all other long haul segments, including all of the FRA route.