"...Cuarón’s most effective decision was to shoot so many scenes on the streets of London, without adding much except graffiti, litter and all-round squalor."
Actually, I lived around the corner from one of the locations in the film, and remember the film crew coming in and cleaning up the pretty disgusting real filth (human excrement, used needles, etc.) and painting over the real graffiti, replacing it with cleaner pretend filth (scrunched up newspapers and the like) and safer pretend graffiti.