Debatable, that one

The
Oxford English Dictionary comes down on the side of -ize as the preferred usage in
British English in the majority of cases. Although it "looks" American (and of course is the standard spelling in American English) and this seems utterly counterintuitive, it is actually perfectly correct British English too. This has nothing to do with us supposedly adopting American spellings either: we aren't, in fact if you read anything from the late 19th/early 20th centuries you'll find -ize all over the place!
Other less definitive dictionaries though would have you believe that -ise is the standard spelling.
I'll put my anorak away now.