I would be interested to understand how unions work in the UK, but In the US, much of their power comes more from regulatory protections than collective labor value and free contracting.
If you don't like it, you can't pick or make another.
By us law, you are usually allowed to quit the union, if you do, you still have to pay the union part of your salary.
In the US, workers can't join or start a union unless 51% of all workers vote to join them/let them.
If all this sounds crazy, polarizing, and like a recipe for conflict, you are correct. I think the European versions of unions makes much more sense, and are closer to free association