The title of the movie "The Sweeney" derives from Sweeney Todd, the cockney rhyming slang for "Flying Squad", which is the name of the branch of the Metropolitan Police that Regan and Carter work for. The Flying Squad specialize in investigating armed robberies and ambushing suspects in the act of their crime. They are frequently armed although not routinely as depicted in this film.
The finale of the film is set in the riverside town of Gravesend. Actually, Gravesend is home to the training centre for the Metropolitan Police's firearms teams. The villains choose to make their escape through the only town in England that is normally brimming with armed police.
Although Regan and Carter are shown driving cars, the real Flying Squad have specialist drivers. All they do is drive, and are trained to exceptional levels. The most senior officer in a car would sit in the front passenger seat, as was the case in the original TV series The Sweeney (1974) in which most of the driving was done by the "Bill the Driver" or other drivers.
One of the conditions of being able to film the shootout scene in central London was that there were to be no gunshot sounds, for fear of alarming people or creating a security incident. The guns used special, newly-developed blank rounds that made little more than a 'click' noise but otherwise allowed the guns to be fired normally, with the sound effects added during post-production. During filming, the actors' cues were provided by an off-camera assistant shouting "Bang! Bang! Bang!" through a loudspeaker.
The trivia items below may give away important plot points.
While in prison, Ray Winstone's character, Regan, stuffs batteries into a sock to use as a cosh. Winstone was in the 1977 movie Scum (1977), set in a borstal (a UK juvenile prison), in which his character made a similar weapon.