Most academic and technical writing in the UK still uses the z form, and the OED and Collins dictionaries tend to prefer it, too: https://en.wikipedia.org/wiki/Oxford_spelling
In popular writing, the s forms dominate - I've not heard the MS Word explanation before, but the most popular UK-produced word processors and spellcheckers in the 1980s (eg. Locoscript/Locospell, Protext/Prospell, 1st Word) tended to come from companies in the Cambridge area or which were founded by Cambridge grads, so would naturally have used the s spellings by default.
> Most academic and technical writing in the UK still uses the z form
'z' forms are generally used for writing for an international audience, it hasn't really caught on more generally than that.
I'm British, but when submitting papers for blind review, always use American spelling for obvious reasons. I suppose I could change it after acceptance, but that would just be pretentious.