Going to take time to sort through all these issues but I assume they won't retrospectively strip work permits that have been previously granted. Also by the time it all goes through many of these players will have been there long enough to get residency. Like I said in the Arsenal thread I expect there will be immediate price hikes for anyone who would qualify for a work permit now the player pool is going to significantly reduce.
On another note though, would not being in the EU mean UK based players cannot get Bosman moves when their contracts expire? Because that was an EU rule wasn't it?