Wait a bit for cheap fares. If you can book at all (the flights will have only just opened for booking and maybe not the whole month yet) it will be expensive. Wait for the airlines to have a sale for that period - not before this summer and maybe the autumn.
As Norman says you will not get there from Leeds/Bradford or Manchester. Heathrow is the obvious choice but not the only one. You could fly via a continental airport (most likely Frankfurt) or via the USA - eg Manchester to LA to Auckland.
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