PC Plod

PC Plod was a derogatory term used for police officers. Investigating a murder committed by an alien on Earth, the Tenth Doctor told Donna Noble that they didn't want PC Plod sticking his nose in. (TV: The Unicorn and the Wasp) Clyde Langer jokingly referred to Captain Tybo as an intergalactic PC Plod. (TV: Prisoner of the Judoon)

Behind the scenes
Plod is a character in Enid Blyton's series of books featuring Noddy. Although the Doctor saying "PC Plod" is the apparent reason why Donna thought about Blyton and Noddy, the connection is indirect. In both cases, the usage is vernacular.