Have you noticed there are variations in sci-fi concepts of the dystopia? There are dystopias that seem to deal with large city centres such as Dark City and Strange Days. On the other hand, there's the suburban dystopia such as Stepford Wives and Fahrenheit 451. And then there's the "pleasure dome" dystopia such as The Island.
I loved Children of Men. I think it is an example of the urban dystopia...with scenes of horrendous warfare in a gloomy urban environment of London...
what makes an urban centre a dystopia?
the concept of inner city misery vs suburbia?
what other authors like George Orwell (1984) writes about the dystopian city?