Airlines price each market depending on what they think can give them the best revenue, so presumably the market from New Zealand has been found to pay more than the market from the UK. It has nothing really to do with exchange rates, in the same way as it has nothing to do with mileage or costs. It's what the marketing team find people will pay.

Consider a carrier who is trying to build a market for themselves from the end you are travelling from, like Korean or United, if price is your only consideration.