We left the Union of the Crowns. It remained a Union after we left it. Our justification for leaving it was the natural law assertion that people have a right to independence from a government they felt no longer served their interest.
There was no union in GB it was a KINGDOM
It's proper name is the "United Kingdom." It was created by the Union of the Crowns, and it was in fact a Union.
Now if you are arguing that the United Kingdom was an "involuntary" Union, then I will point out that ours started out as a voluntary Union, but after "Four score and seven years" it certainly ended up as an involuntary Union.
We were not voluntarily a part of the “United Kingdom”, we had never assented to being incorporated in any Kingdom. We were subjects of the King even though we were not a “Kingdom”.
The colonists did not object to being subjects of the King as long as they were represented in his government. Initially they fought to achieve the rights of Englishmen.
No Kingdoms declared their independence from the UK although Scotland tried.
The South controlled the federal government until it took its ball and tried to go home.