PC Plod

PC Plod was a derogatory term used for police officers. In Perivale, Ace called a policewoman who was arresting the Seventh Doctor "Mrs Plod". (PROSE: Cat's Cradle: Time's Crucible) Investigating a murder committed by an alien in England on 8 December 1926, 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)

In non-valid sources
PC Plod was in Toyland, standing next to Big Ears, when Noddy drove a young boy through town in his car. (TV: )

Other matters
The term comes from the fictional character PC Plod from Enid Blyton's Noddy books, which is why the Doctor saying "PC Plod" caused Donna to think about Noddy.