The United Kingdom has two country codes and uses the exception

Type gov.uk and there is a small standards problem at the end.

Country-code domains were supposed to use the two-letter codes in ISO 3166. RFC 920 said so in October 1984, when it also noted that no country domains had yet been established. Under ISO 3166, the United Kingdom’s assigned code is GB.

The internet uses UK.

IANA’s records give .uk and .gb the same registration date: July 24, 1985. But .uk became the address used by governments, universities, companies and everyone else. The .gb record remains in the root under the wonderfully uncommercial management of “Reserved Domain - IANA.”

In a 2006 paper, ICANN described the arrangement with bureaucratic calm: the United Kingdom had elected to use the “exceptionally reserved” code UK as its primary domain, while GB was effectively inactive.

UK is arguably the better label. Great Britain is the island containing England, Scotland and Wales; it is not the whole United Kingdom, which also includes Northern Ireland. But ISO codes are not a contest for the clearest abbreviation. Their administrative value is that a separate standards body supplies the list, sparing internet authorities from deciding what counts as a country and what its initials ought to be.

That neutrality survived by making an exception. ISO reserves UK so it cannot be assigned elsewhere. IANA keeps GB present but out of ordinary use. The live network, the formal code and the spare code are all preserved.

Standards do not always remove ambiguity. Sometimes they put a fence around the ambiguity that won.