One of the consequences of Brexit is that British people are more limited in their freedom of movement. Whereas previously they could travel, work, retire, settle in other European countries, today the default is that they can only visit the Schengen area for 90 days in any 180 day period and lack rights to work. EU citizens are similarly more limited in what they can do than before, though only with respect to the territory of the UK. (Irish citizens, being part of both the EU and a common travel areal with the UK, are uniquely privileged).
I mention these facts purely as an entrée to my main subject, which is to begin thinking about the positive value of free movement across borders, a topic that is little considered by political philosophers and theorists and is low down the agenda of many politicians, who are more concerned with keeping out the unwanted and security at the border than they are with the liberties of their own citizens to travel, settle, work elsewhere and to associate with people in other countries and of other nationalities than their own. I take it that all of these liberties are valuable to a person and enhance their autonomy for the same reason as the freedom to travel within a country’s borders is valuable.