en-GB means United Kingdom English, a form of English used not only in the UK, but also Commonwealth countries such as Canada (which uses a mixture of en-GB and U.S. English (en-US)) and Australia.
The microformats wiki re-uses the W3C Manual of Style Spelling Editorial Guideline and so generally uses en-US, not en-GB, for page names, and the contents of English-language pages.
There is no "en-UK" language code although it is often used on web pages.
- ASF Bugzilla bug 29517 Correcting en-UK to en-GB
- ISO 3166 English country names and code elements - lists "GB" for "United Kingdom". No presence of "UK" suffix. Also, note that country name is not the same as language.