A key exchange occurs in the third episode of “Gangs of London,” the Sky Atlantic series aired in April this year and now streaming on AMC+; it’s a conversation between Elliot Finch (Sope Dirisu), an undercover cop embedded for two years as an enforcer for the Wallaces, the city’s most powerful organized crime family, and his handler, DI Vicky Chung (Jing Lusi). She wants to pull him out. He wants to move forward: The Wallaces are in turmoil following the killing of their patriarch, Finn (Colm Meaney), whose son, Sean (Joe Cole), is his unstable successor. Elliot surmises that under Sean’s leadership, the Wallace syndicate will fall into disarray, making for easier arrests and prosecutions. “If we’re lucky, this will get messy,” he tells Vicky. “Lucky?” she replies. “For whom?”