I doubt your scenario. If we were doing well and prospering then we would have no need to join - it's not in the Britain temperament for these kind of things (utopian projects), which has been evident since we joined.
Speak for yourself. The referendum showed that 48% of the voters do have the vision to build a better Europe instead of kicking it apart.
We are independent types. So the only reason we would be knocking on their door would be because as I said. We would be begging to join as we basically did in 1972.
Maybe we will be begging to join after the Brexiteers have driven us on the rocks.
I'm sorry but our day is passing. Younger people without the baggage of nostalgia for the days of empire will be taking over and they will build a future inside Europe where we belong building alliances with our neighbours not kicking them in the teeth.