As much as I like some Shane Meadows films, he isn't that original in his film making. I wonder how many of his fans have seen any Ken Loach or Mike Leigh stuff. I've always sort of seen Meadows as a bit of a copycat director in this sense but trying to put a cool and edgy spin on his films which Leigh and Loach have no interest in doing.
And as for the improv, as someone said, Leigh lets his actors create the characters first for weeks sometimes, until they know that person and how they interact with each other, and then he writes the script around it. If Meadows did improv on this, it didn't have the same realism Leigh's films achieve. But that said, Meadows is a mere beginner in this field compared to the other directors however I still enjoyed This is England 86 greatly.