Has American completely cancelled their AKL service? If not, look at GLA-LHR-LAX-AKL. A380 and 787 (or you can do it on all AA metal).
The other option is GLA-LHR-KUL-AKL (KUL-AKL on MH). I'd choose CX over MH though.
Finally, if you want to fly on BA as far as possible you could do GLA-LHR-SIN-SYD on BA then SYD-AKL on QF.