Have you converted your NZ licence to a UK or JAR PPL? If so you're good to go with your sign off with your UK instructor.
As I understand it you can only fly an aircraft of the same jusrisdiction as the licence you hold, i.e. NZ licence - NZ registered plane, UK licence UK registered plane etc.
If you have a UK licence then the UK instructor can type rate you on a UK registered plane. I don't think they can sign you of on a NZ PPL using their UK instructor rating.
I hope that's a bit clearer.