When the euro was first agreed, the UK and Denmark “opted out”. But at that stage that only meant they were not joining at the start. There was never intended to be any long-term form of EU membership that did not include euro membership. The UK did not say “never” to begin with, and all new EU members since the euro began in 1999 have had to commit to joining. Indeed, by 2020, all but five member states of the EU are due to be euro members and Poland is likely to join by then as well, leaving just the UK, Denmark, Sweden and Bulgaria outside.