There were no Flying Squads in Victorian London, but the concept is worth knowing as there are occasional references.
Basically the Flying Squad is a unit of highly mobile officers with radios who can respond very rapidly to major crime, setting up ambushes and the like to take fleeing criminals. They are especially concerned with professional crime, and they are especially useful because they are not connected to the PoliceDivisions but cover a wide area.
You will occasionally see references to Victorian Flying Squads, but these are incorrect; the first FlyingSquad was the "Mobile Patrol Experiment" of 1919-20, from which it has developed into a team of about 180 CID officers today.
See Fido, Martin, and Keith Skinner. The Official Encyclopedia of Scotland Yard. Rev. and updated ed. London: Virgin, 2000.