The correct country code is en_GB. Locales use ISO 3166-1 for country codes. The wikipedia writeup includes:
The codes are chosen, according to the ISO 3166/MA, “to reflect the
significant, unique component of the country name in order to allow a
visual association between country name and country code”.[7] For this
reason, common components of country names like “Republic”, “Kingdom”,
“United”, “Federal” or “Democratic” are normally not used for deriving
the code elements. As a consequence, for example, the United Kingdom
is officially assigned the alpha-2 code GB rather than UK, based on
its official name “United Kingdom of Great Britain and Northern
Ireland” (although UK is reserved on the request of the United
Kingdom).