I wondered this myself. Not that it bothers me as i'm originally from the UK.

All the dialogue is inspired (I guess you can call it that) by how people used to talk back in the day in London. More specifically hints of the "Cockney" dialect peek through in almost every conversation with an NPC. It's just done in a way that I actually find funny. Almost in the same way when an American would do a poor English accent, we just know and find it funny.