New Zealand is an amazing country. It offers pretty much whatever you want to do – whale watching (eg, Bay of Islands), culture (eg, Rotarua), wine tasting (eg, Hawke's Bay), death defiance (eg, Queenstown), cool cities (eg, Auckland), amazing views (eg, everywhere). I've been numerous times, and would go back for more. It's a Bucket List Country.
If you're keen, look at different flight options. For those of us who commute regularly between the Antipodes and UK, 20+ hours is not a big deal, but it's a long flight for the uninitiated, especially if you're in economy ...
Breaking up the flight can be a way to go - it may not be that expensive to fly to a destination in SE Asia and stay there for a night to break the flight and have a sleep. I have done this hubbing through Singapore.
For jetlag, my experience is that the optimal time to land at your destination after a long-haul flight is from about 4pm-9pm, as you have dinner and then go to bed. A lot of the UK-Aust flights arrive at 6am (in both directions!), but there are outliers (eg, Qantas LHR-PER is absolutely perfect, arriving in Perth at around 4pm local time, but it may be too frightening for you). (Edited to add: you'd pick up a Qantas or Air NZ flight over to NZ from Perth.)
Other tips for jetlag are to take melatonin for the first few days, and to have a sleeping procedure – hot shower (to drop the temperature when you get out - mimics sleep pattern), eye mask, etc. That said, jetlag can be a bit random.
I hope you enjoy your future trip to NZ, OP!