NZ no longer operates LHR-HKG, they only have a code share with CX on that route.

If you will be flying enough to maintain Silver or Gold with NZ, you may find Airpoints a good value for the complementary upgrades. If not, the award miles if credited to UA will have greater value.

Also note some aspects of Airpoints are not available for non-NZ residents. There is also no credit card available for US residents.