Please check out this link:
http://www.careerdynamo.com/uk_mba_school_links.html
Have a look at each school you are interested in by clicking their links. I would advise you to think not only about the quality of the school but also about the place you would be staying. London is very expensive- but there are lots of Japanese people around (good support network, food etc but bad for learning English as you will probably spend your time hanging out with only Japanese). Warwick has an excellent business school and is cheaper than London.
If you are applying for a MBA programme in the UK they will look to see if you have relevant work experience as well as sufficient English language ability. Not many students opt to do a MBA straight after graduating from a degree. Are you really interested in doing a straight MBA- there are other interesting Masters courses such as in International Finance, Accounting, Marketing, IT etc.- would these be better for your career?
Also the method of teaching is different from Japanese university/school- students are expected to be much more active in class, freely giving opinions, and importantly not afraid to make the odd mistake. I guess as you are graduating from a NZ uni you should be aware of this already.
Finally, do your research well. Work out how much it will cost you- remember that you will need to pay all the tuition fees plus rent and food. Why pick the UK?- there are good business schools that teach in English that are based in Japan, or what about Australia or the US?
Anyway- I wish you luck in whatever you decide to do.