We say “fall” and the British say, “autumn.” But they changed it, not us. They used to say “fall” in England, too.
I just found out that the same goes for spelling “organize” with an s instead of a z.
Q: Are spellings like ‘privatize’ and ‘organize’ Americanisms?
A: No, not really.
When American spelling was standardized during the 19th century, the consistent use of -ize was one of the conventions that became established.
However, since then, the -ise spellings have become more popular in Britain, perhaps partly as a reaction against the American custom.
Spellings such as organisation would have struck many older British writers as rather French-looking.
The Oxford English Dictionary favoured -ize… and this was also the style of Encyclopaedia Britannica (even before it was American-owned) and formerly of the Times newspaper.