With some long hauls, it can be cheaper to fly from the destination to the UK and back than in the reverse direction. South Africa is one such,. HOWEVER, you cannot book from the distant end to start the journey here!
I think the idea of checking the total amount to be paid and considering a single bank txfer at a rate that you know in advance, sounds worth exploring, if you have the time to do so.
Air NZ have a consistently good reputation and their Y and PE is meant to be amongst the best in the world.