.gb and .uk are both for the same country. Also .io is associated with that country as well. And politically. .gg and .je also belong to the British. Those two are not even countries to begin with. They are "crown property".
And since no .io domain ever was registrated to anyone living there, on account of the area bein non inhabitated (by civilians) before the internet arose, the domain registration in this case was dubious from day 1. For all intents and purposes it was used as a general tld from day 1.
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Oh, and the nic for io is this one. And they also do .ac and .sh, two more British tld.
Internet Computer Bureau Ltd (ICB) is the country code top level domain manager and registry operator for the .IO, .AC, and .SH top level domains and is located in the United Kingdom.