On paper at least, the UK and Denmark do have special opt-outs from the Euro while all the other EU states are legally bound to join it. In reality it's likely

On paper at least, the UK and Denmark do have special opt-outs from the Euro while all the other EU states are legally bound to join it. In reality it's likely that several of the non-Eurozone states will simply never join it and there's nothing to ensure that they ever do.