I think you could make the argument that Britain rejoining makes the EU look good - if the UK leaves and does better, it might give other countries the idea that they'll be better off leaving too. But if the UK changes its mind and comes back after not even 10 years of being out, it might send the message to others that you're better off staying the course.

Especially if we're willing to come back for a worse deal than we had before we left, which I'd assume would be the case. We pissed away an amazing position within the EU, I'd be very surprised if they were willing to let us just plop back into our old role.

Would the EU even allow Britain back in at this point?

Can only guess. But If they even considered it. I expect they would want a clear majority vote. 66% or above would be my guess.

I imagine the terms would be much less favourable if they did

Likely the same as any new nation joining.

I imagine the pound wouldn’t be allowed anymore for a start. You want back in? Adopt the euro.

Like many new nations joining. That would be less simple. As Joining the Euro has economic requirements. I can actually see the UK promising to adopt the Euro one day. Then never really working to do anything beyond accept it in the UK along with GPB. Because joining the Euro is always an eventual requirement for new nations, rather than an immediate one. If Europeans can spend euros in the UK. I can't actually see the EU forcing the UK to join any time soon.

That is of course ignoring point one is unlikely to happen.