What's the connection between Schengen and Brexit?
The UK is not and has never been in Schengen.
I guess GP is taking about free movement of EU citizens, but that has nothing to do with Schengen.
>free movement of EU citizens, [...] has nothing to do with Schengen.
Did you mis-speak?
One of the things leading up to Brexit was politicians claiming we couldn't police our borders without getting out of the EU. That was of course false. Almost the opposite in practical terms.
Presumably, if UK were to return to the EU we would do so without our past veto, and as part of Schengen. That makes it less desirable.