No Brexit:
UK could remain in the EU but on worse terms, says Guy Verhofstadt. If Britain changes track and decides to remain a member of the European Union it could lose perks such as budget rebates and opt-outs, according to the European Parliament’s Brexit coordinator.
https://www.independent.co.uk/news/u...-a7790886.html
So if the UK wanted to rejoin the EU it would have to submit an application for membership as if it were applying for the first time.
That probably means signing up to the Schengen area on passport-free movement and, importantly, committing to joining the euro. It would also mean recommitting to the goal of “ever closer union”.