Possible Clues:

• Brit's idiot box
• Flat screen?
• Familiar name for TV cop in seventies
• Box 19 having article inside swapped with second in 14
• “[…] if a monster were out roaming northern England we’d have seen it on the ____” (An American Werewolf in London)
• BBC watcher's set
• Television
• Box like William, the national hero?
• William's unknown box
• Sesame Street worrywart