Like Denmark, the UK negotiated an opt-out for joining the Euro. But this opt-out would not automatically carry over if the UK rejoined. That means the UK would have to adopt the Euro just like any other member but they would have to meet the convergence conditions and part of those conditions is joining ERM II (exchange-rate mechanism) which is voluntary. So they could stall the adoption indefinitely by not joining ERM II just like Sweden, Poland, Czechia, Hungary and Romania.