I also recommend the Air NZ product. With their aircraft having a refit resently the economy cabin is really comfortable. The PE is great, and the business suberb, but the economy is still very good. I'm 180cm and found plenty of legroom to spare.
The flight from LHR wil arrive in AKL, then you will have to transfer to the domestic service which is average, but it is only 1:20 flight down to Christchurch.
Jetstar are the new LCC flying CHC to OZ.