What kind of concessions would the EU be likely to levy? It feels weird to put a member state at an economic disadvantage simply because their politicians are dumb shits.
Together we're all stronger, so as a EU citizen I'd welcome them back. However, say goodbye to the GBP, it's Euro for all of you, and no more special rules, it'll be a regular old member state.
If they were to apply to rejoin, as well as joining the Schengen and the euro, I'd want them to have a minimum 60% in favour in a referendum (not just in the polls). Something like a membership of the EU, a political project, requires strong public support.