When it was a member of the EU, the UK had an opt-out for the Euro currency (as well as other accommodations).
It's likely that most of those accommodations would be a requirement for the UK to rejoin the EU.
It's likely that most of those accommodations would be a requirement for the UK to rejoin the EU.
New EU members must agree to join the Euro as soon as they meet the Maastricht Convergence Criteria, one of which is that their currency must have been part of the Exchange Rate Mechanism for two years... but there's not actually any requirement to join the ERM!
Countries including Sweden, Poland, and Czechia have all taken advantage of this to retain their own currency - and the UK could do the same.
The other main opt-out held by the UK was around the Social Chapter, but that had already been given up in the late 1990s.