Armstrong, 29, is best known for playing the titular hero in the BBC series "Robin Hood" for three seasons, ending in 2009. He has also appeared on TV's "Teachers" and "The Street."
The BBC said the police citation stemmed from an incident in which the actor had been drinking at a family christening, then became belligerent with a taxi driver taking him home afterward.
Armstrong misplaced his jacket and had no money to pay the driver, then allegedly attempted to punch him, spat at the car and swore at police officers who arrived at the scene, the British broadcaster said.
The actor reportedly pleaded guilty to the charge of being drunk and disorderly, and agreed to pay a $143 fine and court costs.
He denied, however, spitting at the car or trying to punch the driver, the report said.