It’s normally unbroken use abroad that will block a roaming sim. Could you not have two 'Three' sims that you alternate between - make a few calls each month in the U.K. then send out to NZ, send the old one back to the U.K., use it on the home system, rinse, repeat?Roaming in NZ with Three is limited to two months.
I'd expect to go out there with a Three SIM and for the thing to work for 2 months.
Are you saying Three was a bad choice?