The Season 6 finale of BBC police corruption drama “Line of Duty” snared over 15 million viewers — the century’s most-watched UK drama episode.Season 6 is now available on streaming service BritBox, with familiar faces Martin Compston (DS Steve Arnott), Vicky McClure (DI Kate Fleming) and Adrian Dunbar (Superintendent Ted Hastings) returning to investigate a murder case with sinister and far-reaching implications.