The country codes used for ccTLDs are arbitrary, I thought: they're roughly consistent with ISO 3166-1 alpha-2 codes but not exactly. Are there precedents to suggest a country can only have one ccTLD?
With the demise of X.400 e-mail and IANA's general aim of one TLD per country, use of .gb declined; the domain remains in existence, but it is not currently open to new domain registrations.
https://en.wikipedia.org/wiki/.gb
NB I was curious as to whether .gb would be strict and exclude NI - who get to use .ie if they want...