The United Kingdom was a Union??? The fact you are asking the question demonstrates that you didn't bother to follow the link I provided which showed you that yes indeed, the UNITED Kingdom, was a Union.
But let's do a trivia question. What is this flag called?
None of the vassal states joined in voluntarily.
What does voluntary have to do with anything? If you can voluntarily join, but not voluntarily leave, it isn't really voluntary is it?
More like a Mafia Family.
My point is the UK didn’t exist.
It was the British Empire. The flag you posted originated in 1801, after the American war of independence.
Now tell me again how the Democrat slave owners attempting to leave the US are the same as our forefathers fighting for freedom. Keep in mind that Washington and others were slave owners when they were British subjects. Slavery continued throughout the British Empire until 1853.