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I use both spellings on my SEO pages, with more "iz" than "is".
As a graduate of London University , the third (chronological) university in England, I'm tolerant of both opinions. Those who have studied at more recent Englsh and Welsh universities may, of course, have other things to say on the matter, as may graduates of Scottish universities (both ancient and modern).
Graduates of the University of Life may also have equally valid points of view.
My own usage has been determined by the word processsor I have used. M$ Word Insists on "ise" as the British English ending, so I use that, these days, to minimise hassle. In the days when I used a DOS machine, my preferred word processor was Smartware, which went for "ize" and therefore so did I.
Its a mix of British and US I guess.
The jury's long been out on -ize and -ise. Of the two, -ize is probably the more traditional English spelling, but -ise is much more commonplace these days. I personally use -ise because it looks better, and so do most English newspapers. But it's actually an Americanisation.
A good way to check standard common English useage is to simply see how the BBC spell it (although they insist on writing Internet as internet - which I think is incorrect)
Now that you're all fast asleep, I'll sign off.