We weren't "allowed" to stay out of the Euro, it's each nation's right to choose whether to adopt the Euro or retain their own currency.

Not true, might have been the case once, but not now. Any country joining now must adopt the Euro as it's currency. That came in with the Lisbon Treaty that wasn't a Treaty because a number of countries raised objections and threatened to use their vetoes, so they called it something else under some sort of backdoor EU scheme and got it through (Gordon Brown could explain it to everybody) - that's the EU for you - about as corrupt, inefficient and unaccountable as can be.