The Oxford Guide

Considered by most residents and students of the city to be a menace, especially when mounted on open-topped buses and equipped with microphones.

Apparently they operate as a bit of a cartel - newcomers to the tour-guide market are strong-armed back out again - anything from them going up to the tourists who've just been guided and picking holes in the guide's rhetoric to calling the police saying they've seen someone without a license, etc. They even have their own guild!