Is en_UK an illegal locale?

Question

So far I had always used \'en_UK\' for British English. Today I got an error when using it with Zend Framework because the locale is not included in the long list of recognized

Answer 1

Imperfect implementations

Starting in 1985, ISO 3166-1 alpha-2 codes have been used in the Domain Name System as country code top-level domains (ccTLDs). The Internet Assigned Numbers Authority currently assigns the ccTLDs mostly following the alpha-2 codes, but with a few exceptions.[9] For example, the United Kingdom, whose alpha-2 code is GB, uses .uk instead of .gb as its ccTLD, as UK is currently exceptionally reserved in ISO 3166-1 on the request of the United Kingdom.

According to wikipedia http://en.wikipedia.org/wiki/ISO_3166-1_alpha-2.

So apparently it's an old thing that has stuck or something like that.

EDIT: martin clayton found a better answer.