Pretty much half of the things that are everyday in New Zealand are not available at all in the UK, or if they are, they must be purchased via little-known websites that import it directly for you. I could go on forever. Here's a few of examples off the top of my head, though.

Colby cheese
Mountain Dew
K-Bars
Mince and cheese pies (I haven't looked at this in-depth, I just thought of it now, actually)
Nutri Grain cereal
Maggi onion soup mix / Nestle reduced cream and, because of these two, a good onion chip dip
Fresh Up
Jellytip ice cream