[closewith]

Although interestingly, Northern Ireland and the Channel Islands have the ISO country code GB, but Gibraltar (GI), the Isle of Man (IM), the Falklands (FK), and many others do not. Some are Crown Dependencies, which I suppose is understandable, but others are part of the UK.

[BoxOfRain]

Gibraltar and the Falkland Islands (among a few other places) aren't crown dependencies or part of the UK, they're the vestigial remnants of the British Empire that rejected full independence for various reasons. Until around the time of the Falklands War they were still called crown colonies but they're now called British Overseas Territories. They're not like France's equivalent where many are integral parts of France.

[closewith]

Even then, the Channel Island are a Crown Dependency and use the GB ISO code, while the Isle of Man is also a Crown Dependency and has its own ISO code.

The British Indian Ocean Territory and the British Antarctic Territory are both British Overseas Territories, but unlike the Falklands, Bermuda, etc, do not have their own ISO codes.

So the determination is more complex than that.

[Symbiote]

These have codes GG, JE, IO.

Claims of Antarctic territory aren't recognised internationally, so the codes were removed, but it used to have the code BQ.

[dan1234]

There's a CCP Grey video which take a decent stab at untangling everything:

https://www.youtube.com/watch?v=rNu8XDBSn10