When BA operated to New Zealand
In the says when flights operated routings like LHR/BAH/BOM/SIN/SYD/BNE or LHR/DEL/BKK/SYD/AKL crews were based in Sydney. Certainly, in the case of FD crew, they were UK employees. They spent 6 months in Sydney and (from memory) operated everything east/south of Bangkok and Singapore. Using London based crews to operate all the sectors would have had people down route for up to a month at a time. With the advent of the -400 and the reduction in number of en route stops, combined with withdrawl from New Zealand, the current pattern was established.