Actually this is a common misconception. BrE allows either -ise or -ize, whereas AmE only permits -ize. The -ize spelling in BrE is called "oxford spelling" https://en.m.wikipedia.org/wiki/Oxford_spelling

When I write code or here on HN, I try to use Americanisms... But when writing longer form or official things, I use British English.

I doubt I'm the only one.