UK spelling dictionary - teach OS X *all* -ize spellings
I found a good-enough fix for this.

I copied the Oxford English dictionary -ize words from here:

http://en.wikipedia.org/wiki/Wikipedia:Manual_of_Style/Spelling/Words_ending_with_%22-ise%22_or_%22-ize%22

and pasted them into

/Users/danbrowne/Library/Spelling/LocalDictionary

This seems like a fairly comprehensive list. Anything which is missing I can add by hand.

One remaining issue: Apple doesn't seem to be able to intelligently conjugate these words, but using copy-paste on the list (ize -> izes, izing, ized, etc.) sorted that out.