If anybody is dumb enough to think enough Air New Zealand pilots would be able to replace even ONE DAY'S WORTH of a BA schedule anytime before 2021 then they shouldn't be at the front of an airplane let alone in charge of one. Even if you could find 500 NZ pilots who were suddenly willing and able to travel to the other side of the world, dragging their families, to an airline like BA whose reputation is in the toilet, how exactly would you get them recruited, sim checked, OCC course, HR bull**** week, visa done, licence conversion, line checked (20 - 40 sectors)... 6 months to a year at the minimum, if even possible at all!
Jesus christ, call their bluff, they can't fly planes without you, and you are not as easily replaced as they want you to believe, especially once it goes public that they are trying to replace all of you wonderful Nigels with Johnny Foreigner, does anyone remember Brexit? The Sun/Mail etc would destroy them for it.