I am going to be doing flights between the UK and New Zealand at least once a year for a while. I am trying to price up some flights to help with budgeting - and flights booked OUT of New Zealand are always considerably more expensive than flights booked OUT of the UK.

Why is this and is there a sensible way to make sure I am using cheaper flights? E.g. Can I buy returns out of the UK with flexible 'return' dates, and then change the return date when I know I want to head back to the UK? Any flaws with this plan?

E.g.
Singapore CHC-LHR-CHC for two people is NZ$5,200, ~£2,600
Singapore LHR-CHC-LHR for two people is £1,600

Emirates CHC-LHR-CHC for two people is NZ$5,200, ~£2,600
Emirates LHR-CHC-LHR for two people is £1,549

Other airlines are similar.

Thanks for the input. I have no problem paying for flights on either UK or NZ based cards.