Singapore Airlines is the quickest, 22 hours 40 minutes.
Considering that even 6 or 7 hours feels like forever to me squished into an economy class seat, that is not exactly huge fun.
I've only done the return flight to London or Western Europe 3 times. If I can scrape together enough money for a family holiday I might have one more return flight left in me.
I've flown from Sydney to Washington DC a couple of times as well. That's 20 hours. If I can ever afford that holiday I think we'll do the full loop, Sydney to London, then the US, then back to Sydney over the Pacific.
Christchurch to London is even worse, 28 hours is the quickest flight. Apparently it's about the same distance flying West or East from NZ.