Learn English – “Normalise” or “normalize” (British English)

There is a suffix that is written only as -ize in American English and often -ise in British English (but not always, as ShreevatsaR points out in the comments). This suffix attaches to a large number of words, thus the s/z alternation shows up in a large number of words. Citizen does not have the -ize/-ise suffix.

My 1983 Chambers gives the -ise spelling first, which seemed reasonable to me because I think the -ize one looks 'American'.

But my 1998 Chambers gives the -ize spelling first, so I guess I have to assume they've changed their tune in the interim. Note that this change is reflected for all relevant words in the later edition. As this article points out, almost all British 'authorities' apart from OED (Oxford English Dictionary) have now adopted/endorsed the American standard.

For what it's worth, NGram suggests that ever since the word[s] really started to take off in the 60s, fantasize has been more common even in British English. I think maybe the fact that it was so rarely used before has allowed Brits to take the 'easier' path of simply copying American usage; there's less historical usage to be swept under the carpet, and it seems to have been more used by Americans earlier on anyway, so it's only fair they get to say how we spell it.

Thanks to @Peter Shor for commenting that the 'original' British form was phantasise, as can be seen from this NGram. In light of that, perhaps the 'pseudo-original' fantasise could be seen as an abortive attempt to mimic (a dying) 'standard' British spelling for historical reasons. Which failed/is failing because there simply isn't enough history to sustain that position.