The airlines apply their own exchange rate which they only change (usually) once a week. That rate would apply if you bought the ticket through someone in the UK (having had a quick look at the Air NZ website not sure how you force that other than using an agency or phoning Air NZ). The point being that you can get a quote in Pounds to compare against the Air NZ Dollar fare. Then you can phone your credit card provider ask what foreign currency changes/rates they apply and then come to a decision. If you don't usually make large purchases by phone or the web I would be prepared for your card provider to say "no" when you push the purchase button; don't leave the website just get onto your provider immediately and work with him to get it through.
Of course the other option is to go to your bank and transfer the money to NZ and let them make the purchase. I don't know what banks charge for that these days.
One of the Middle Eastern carriers (Emirates?) will probably get you to Gatwick. For all that people seem to like Emirates I would still go for Air NZ they are good.