Well, maybe the EU won't give them a choice if they want to join. Every new joiners adapts to the euro, why should the UK be special?

There is already Poland and Hungary who piss everyone, no need for a third

When we take them back there needs to be clear repercussions for leaving though. We need to disincentivise going back and forth about it.

As a starter any trade or economic agreements made, that are touching any aspect that is normally negotiated by the EU as a block need to be revoked in their entirety. Also i think some form of compensation for the damages caused to the EU by brexit should be paid.

Otherwise going back and forth will be a wedge happily driven by Russia, China and others.