My POV as a Western Canada Airpoints member, is the ONLY reason to accrue to NZ is to attain GE and have easier access to upgrades, whether RU's or APDs.
The only way I can earn SP or AP is BIS with NZ or an airline partner.
My anecdotal success rate for upgrades clearing is 75%. This includes many wives, children, nominees.

With regard to customer service when things turn to custard, the app, the website, call centre, I rate NZ with an F.
All are antiquated and/or abysmal.

My comparison is Air Canada where I am an E50 (rough equivalent of Gold with NZ), where phone is answered within a minute, changes made with zero service fees.
AC's app and website show so many more options than NZ's for the same journey, as does UA. It is easier to book ex NA flights on NZ metal using AC.com or UA.com, that is unfortunate.

If I were you being UK based, the ability to buy C/J or F on much better carriers whose business class does not resemble a herringbone milking parlour filled with coffins, I would accrue to SQ or BA to enjoy an awesome customer experience.