Ed West, co-author of he ASI's recent 'The Border after Brexit' paper, wrote a piece for the Spectator titled: In defence of small nation states.
The Spectator notes:
As I argued as a co-author of a recent paper for the Adam Smith Institute, there’s no reason why a frequent American visitor can’t swipe into Britain like he can swipe into the Tube. I’d go further and say there’s no reason why an American can’t freely live and work here. The social costs of free movement between rich countries are very minimal, and in future there will probably be increased competition to attract the small pool of very talented people. It is feasible we could have open borders not just with western Europe but also Japan, the US, Australia, Canada, Singapore, South Korea, New Zealand and, if and when its median income reaches a certain point, China. So there’s no reason why a world of 500 or even 1000 states would slow down the necessary free movement of skilled labour.