Unfortunately this is trickier than you might imagine. For folks in Australia, it will overall be cheaper for you to just buy the Mini-Book Two-Pack and pick up Soonish in October locally. It will have the UK spelling (or as we keep being reminded by some of you, the correct spelling).
If you are in Canada and you want the UK version, you're pretty much in the same boat. The short version is that the "US Edition" is really the North American edition. If you want the UK edition with its "correct" spelling, your best bet is to buy a copy online at one of the sellers listed on our UK page.