There is not much coastline left which is suitable for roosting at night and gull numbers are way beyond what they would normally be without human offal to eat. A quiet paddock inland is ideal and much safer.
They also move inland when there is a storm on the way.
Most common gulls don't migrate as such, merely drift with the seasonal food sources. Many are now stationary for reasons above.
Despite being gregarious they are quite crappy about their immediate space and can kick up a hellofa row.