If the UK did join the eurozone, it would be by choice. All you have to do is promise to join it. Then just sit on your heels. No one will actually make you. Case in point: Poland.

Since the convergence criteria require participation in the ERM, and non-eurozone member states are responsible for deciding when to join ERM, they can ultimately control when they adopt the euro by staying outside the ERM and thus deliberately failing to meet the convergence criteria until they wish to.

https://en.wikipedia.org/wiki/Enlargement_of_the_eurozone