The Wire star saw off Daniel Rigby and John Simm to land the Best Actor honour at the London ceremony after wowing judges with his turn as notorious British serial killer Fred West.
Watson, who played West's social care volunteer in the show, was named Best Actress, and was hailed by judges for her "subtle, nuanced and utterly compelling" performance. She beat This Is England star Vicky McClure and Ruth Negga, who played Shirley Bassey in the biopic of the superstar singer.
Sir David Attenborough's hit documentary series Frozen Planet lost out in the Science and Natural History category to Mummifying Alan: Egypt's Last Secret, while Luther, starring Idris Elba, was named best series.
The international award went to U.S. sitcom Modern Family, beating Australian series The Slap and Danish crime thriller The Killing.
The ceremony was hosted by Welsh funnyman Rob Brydon.