Very interesting problem indeed, as someone studied Political Science, it strikes me how much of a trouble May is in. This border problem is completely crazy, very complicated and has various factors.
If I was someone carrying British passport, I wouldn't vote for Brexit but still I can understand why majority of the population wanted out. Reasons are most probably stated on this thread. I just want to state that I really feel bad for PM May, she is not among my fave politicians by any means but she is in a big trouble about decision making and bravo to her for still trying her best (it seems from a far)