A key problem here is that, according to the treaties, the euro is the currency of the EU. Every new member commits to eventually join the euro (Scotland would have done if it had voted for independence) and only the UK and Denmark have a permanent exemption from this demand. Others like Sweden and Poland can delay joining but the EU treats that as a temporary aberration, not a lasting situation. Even those rules designed to protect the non-eurozone member states, like double majority voting at the European Banking Authority, are designed to lapse in the next ten years.