Actually it doesn't matter - either version is correct, and in fact English spelling used organize long before it was adopted by the US.

http://blog.oxforddictionaries.com/2011/03/ize-or-ise/

In fact it appears more to be laziness on the part of the UK english. Whilst the 'ize' form is derived from the greek endings, certain english verbs had to end with 'ise'. As we were too lazy to remember which ended 'ize' or 'ise' we just took the pragmatic approach and used 'ise' version for all of them.