When we proofread documents to UK English, we always change the suffix -ize to -ise. I decided to look into the history of this.
'Oxford spelling' uses -ize as a suffix in preference to -ise. Example words are organize, privatization, realize. There are, apparently, about 200 verbs that are affected. The basis for the use of -ize comes from the Greek origins of words, the root being -izo for -ize words.
Nevertheless, -ise has become the preferred usage in UK English, from sometime in the 1990s. Now, -ize is often thought to be US English and incorrect UK English, though this is not technically true.
Words that use the suffix -yse are not affected by this argument as they come from a different Greek root. Thus, -yse IS UK English, and -yze is US English.
It's enlightening stuff.
So, although it is not necessary to change -ize to -ise for UK English, the key (as always, in proofreading) is consistency.
There's a good full article about Oxford spelling to be found here on Wikipedia.