There is effectively no conversion from a foreign professional flight crew licence to a UK or JAR licence. You must do all 14 or 15 written examinations and two or three flight tests.
The general attitude of the UK CAA and of some UK pilots is that a professional flight crew licence issued by a country outside the UK is a "corn flakes packet ticket to terrorise the skies". There seems to be a pervasive "not made here, cant be any good then" attitude.
To obtain a UK or JAR professional licence will cost at least NZ$50,000.
The UK licence is not so much about the safety of flight, but more about protectionist employment policies.
Best of luck with it.