Won't happen, that is Remain groups trying to make Leave look pointless, but is bullshit like most claims.
That's not the question of the referendum though, is it?
Problem is, the Leave side don't actually have a coherent plan in place so we're left guessing what would happen. They've said we'd have a deal like Switzerland or Norway in the past yet both of those countries have had to accept free movement of people in order to maintain trade relations. The Swiss system is actually worse because they signed up to Schengen. Even the points system they're now pushing doesn't actually have any substance when you look at it in detail and doesn't then tackle the trade and economic issues that would then arise.
Even the SNP had a plan for the Scottish referendum. I just don't understand how people can accept this level incoherence and poor planning when the Leave politicians have been pushing this for years.