Interesting idea - could work well. The flights from UK to (east coast) Aust/NZ are about 23/26 hours respectively, if done with a reasonable connection. So if your body clock was already at GMT+12, you'd essentially go through a full 24-hour cycle on the trip and arrive nicely aligned with the local time.
For travel from UK to NZ, I daresay our moderator TightSlot will be along shortly to give you every conceivable detail, but in the meantime I can say that in my own experience the two long legs are better than going to Sydney or Melbourne and then doing the 3-hour hop across the Tasman. If you do the latter, you essentially end up doing two long legs and then a short one - even if the second long leg is only 8-9 hours rather than 12, it's not that much difference (it's still a tedious long bloody flight) - whereas adding another 3-hour flight (with all the hassle of the layover, etc) really makes a difference. Of course, if you actually want to stop in Aust for a while, then this option is perfectly sensible.
If you're going J-class, I'd suggest Singapore Airlines via SIN, Cathay via HKG, or AirNZ via HKG or LAX (you can go either way around with AirNZ, or do a round-the-world). If you want to go via Aust, then Emirates would be your best bet, although Qantas will also work (if not on strike).
Let us know how you get on with the body clock synchronisation.