I think the word that gets bandied around is 'gritty'--albeit exaggerated.
Morally ambiguous characters in that fag-end, tawdry London that we inherited from underinvestment and the Luftwaffe. Although since presented/parodied as emblematic of the era, Regan, the anti-hero protagonist, is actually supposed to be an atavistic throwback in the setting--the one whose methods are ill-suited to the winds of modernisation blowing into the police force (see the Operation Journeyman thread), but he's perfectly adapted for the rough parish that he polices.
All the genuinely 'good guys' are victims: working-class Graham Greenes with more out-and-out atheism than existential and religious doubt--lots of psychologically damaged characters. It does give a great sense of environment: strip clubs, boozers, caffs, warehouses, bookies' shops and dogs' tracks--and a proper cast of villains with nicknames and memorable modi operandi. Nobody ever seems to get called by their Christian name unless it's showgirls or hookers, in which case they're probably fake. Everybody smokes all the time.