Some Brits can be particularly snippy around spellings and grammar, as anyone who has used the word gotten in their presence can ably testify.
“We invented this language,” runs the argument, “so anything you did to it after the War of Independence can only be considered a decline in standards.”
So sometimes it pays to have your defense ready, especially if you’re about to get pinned to the wall by a withering look, after spelling organize with a z. That’s z as in zed: not zee.
So, the first thing to say is that the -ize ending on verbs is the most commonly acceptable version in the world. In British English, either version is fine. Yes, generations of readers and writers have grown up being used to reading realise with an s. But as either version is accepted there, and in America only one is, statistically the z wins. The Oxford University Press uses -ize endings in their style guide, but the Guardian does not.
And part of the reason both endings are acceptable is that -ize predates -ise, back to a time before there even was a British or American English. The Oxford English Dictionary cites an example of realize that goes back to 1611, and an organized that dates back to 1425. The first recorded realise with an s isn’t until 1755. And this was during a period where it was not considered important that spellings were uniform.
Once things started to become codified, a struggle began in British English. There was a general understanding at first that -ize would be the way to go, because Greek-derived verbs use the proper Greek endings—-izo and -izein. But there were also verbs where the -ise ending is part of a longer suffix, like -cise, -tise or -mise—as in advertise, advise, exercise, compromise and so on. So some words were Anglicised with the s (like Anglicise), and others—capsize, prize, seize—are still spelled -ize no matter where you may be.
So, this is the opposite situation to that presented by Noah Webster and his dictionary of simplification. The Brits decided to change the language in their own unique way, and then couldn’t decide if they liked it like that or not. Should any of them have the nerve to get snippy about it now, you have my permission to give them what for, until they apologize. With three zs.
Oh, and you may also like to know that gotten is a perfectly acceptable past participle of get and that both got and gotten are traceable back to Middle English. The problem isn’t that Americans developed the word; it’s that the British stopped using it.