Spelling guidelines.
Follow standard British spelling, as prescribed by the Concise Oxford English Dictionary or the Oxford Dictionaries website. (If you don’t have access to either of these resources, set your online spellchecker to “UK English”.)
If the Oxford dictionary lists alternative spellings, use the preferred form — the one that is listed first, not as “(also …)” — for example, “judgement” (not “judgment”) and “programme” (not “program”). Note, however, the following exception to this rule.
Even though it is not the Oxford Dictionary preferred form, use the suffix “-ise”/”-isation” rather than “-ize”/”-ization” — for example “globalise” and “organisation” (not “globalize” and “organization”). The “-ise” form is preferred by British-English mass media and general publications (and is used by GOSG and gov.uk). Furthermore, this maintains consistency with words that always use the “-ise” form (such as “surprise”, “comprise”, “exercise”).
However, do not alter the “official” spelling if an entity uses “Organization” or “Globalization” as part of its name.