Ive also got a NZ PPL and was looking to have it "converted" to a UK license.
There is a section in LASORS which deals specifically for this: ie. PPL license from an approved JAA country into a UK PPL.
I think that you would have to take air law, comms and another subject at ppl level, as well as a skills test, but if youve done the xcountry qualifier then you woudnt need to do that again.
Good luck anyway