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.