Zombie movie that i can't remember the name of. Discription is in details.?

Story is in england. The country was divided in half with some kind of wall and automatic turrets. One side is like modern and was doing some slavery stuff. the other side was meant to die out but some survived the disease and become man eating punks and sluts. There there was a bunch of middle evil knight who want to take over because they think they are more pure or some sort.