So basically you are wanting a LHR-AKL return in premium economy, with the return routed via TYO.
There doesn't seem to be any published fare by NZ that allows this with the TYO-LHR segment on BA.
If you want to travel LHR-AKL, AKL-TYO on NZ and BA TYO-LHR then there are various combinations of fares that allow end-on-end ticketing to construct this routing, but they are not cheap and some segments will need to be economy or business.
There is the option of LHR-TYO-AKL on BA/JL/NZ return AKL-TYO-LHR on NZ/BA.