GK> This leads me to think that "Great Britain" is the name of a political GK> entity as well.
Thanks for pointing that out! In fact I'd never read the Act of Union and wasn't aware of the original name, nor was I ever taught it at school - so I've learned something this morning!
However, I did say that the correct title is "The United Kingdom of Great Britain and Northern Ireland" (as on my passport) which is shortened to the "United Kingdom". And one can talk about "Great Britain" in a general way but it doesn't, these days, necessarily define exactly what we're talking about in terms of a political union. The Act of Union refers only to Scotland and England.