Guys the deal is this:
You can quite happily fly in the UK in any aircraft that you are rated on your NZ licence with certain restrictions - day, VFR, private, only in UK airspace etc. check out lasors. For example I am rated on C172s and supercubs so can fly these aircraft in the UK (as long as they are G-reg) - this is without converting or doing anything (aside from obviously getting advice/check flights with an instructor, briefing on differences in airspace etc.)
As for converting, as you would need to do if you want to fly other aircraft for example, it's simple: less than 100 hours total time you have to take all the theory exams (including RT and practical), a skills test and get a JAA medical. With more than 100 hours it's just Air Law and Procedures, Human limitations (for some reason, never understood that), and obviously RT and practical. And a skills test and get a JAA medical. It's all in lasors.
I got a NZ PPL(A) with Jackie Frame at Hanmer Air a couple of years back. A small set up but I had a great time and would recommend it to anyone. Went back this year and did some flying out of Fielding was also great, they have a beautiful supercub there.
As has been mentioned before on here, Irv Lee has an excellent site regarding conversions etc:
http://www.higherplane.flyer.co.uk/faq.htm
hope that helps.