Generally, "alternate spellings" are misspellings. <grin>
And while we explicitly accept both British and American spellings, that policy generally refers to content. Titles, particularly titles for works, are something we need to be a little fussier about because ten years after we forked from TV Tropes we're still cleaning up a million redirects created by people there who couldn't be bothered to check that they were using the actual name of a work as opposed to what they thought it might possibly be. The ability to create a redirect to the right page name is no excuse to use the wrong one just for convenience.
That said, I did do some quick research before deciding to reject this redirect and I could not find a British release of the film using British spelling. If I turn out to have simply not cast my net wide enough to find it, I will gladly un-reject your edit, because then that will be an official alternative.