None of those are the same country. They are distinct countries.
And the gb / uk thing exists because of the northern ireland situation. Its an exception and a very old one and gb does not refer to uk, but uk does include gb.
Each distinct political area has one tld.
While .io might not have been registered by an inhabitant, the country code it is sourced from WAS used by people living there and continues to be used officially today.
The rules are clear. only country domains can be 2 letters, to match the 2 letter country codes they are based on. if we allowed other 2 letter domains, a collision could easily occur. for example, if Scotland became in independent country, we already have an issue as the obvious 2 letter domains (.sc or .al) are both taken already (sechelles and albania) so something like .sq would likely be used. but we only have even that less than ideal option because people can't otherwise use .sq .
