I guess Rìoghachd Aonaichte (the United Kingdom) is not interchangeable, wordwise, with Great Britain?
That's correct. They're not interchangeable - Great Britain is part of the United Kingdom but not the entirety.
Oh, chan eil? Bha mi ag ionnsachadh an-diugh!
But isn't it though? -- as much in the UK as is Scotland....currently...
No, the Isle of Man is not in the UK at all, never has been. It is a crown dependency, much like the Channel Islands, where the Queen rules as Lord of Man.