> The Euro is not a requirement (obviously) of being in the EU.

They negotiated an opt-out. That's likely to be much tougher if there's a second go around.

https://eur-lex.europa.eu/EN/legal-content/summary/united-ki...

> When the Maastricht Treaty was concluded in 1992, the United Kingdom was granted an opt-out clause, meaning that it was not required to participate in the third stage of economic and monetary union (EMU) and consequently introduce the euro.