CO19 is a rather rubbish acronym for Central Operations Specialist Firearms Command, the metropolitan police armed response unit. CO19 is the equivilant to the American S.W.A.T unit. Their main weapon is the MP5SF - the same used by British S.A.S except limited to Single Fire. Less commonly they also use the G36KSF and the G3KSF. Their main sidearm is the Glock 17.
CO19 is used mainly for detaining potentially armed suspects, although they are trained by the SAS to carry out hostage rescue and are also trained in the use of pyrotechnics, distraction devices and absailing. However, higher priority hostage situations will usually require the SAS who are far more proficient at this due to extremely rigourous training.
CO19 has often wrongly been criticised by the British media for a long run of mistakes, blown out of proportion in order to 'sell the story'. Such incidences included SAS officers that train CO19 complaining about the "gung ho" attitude, citing a training excercise where they surrendered their weapons as CO19 trainees entered the building, but were still shot at (using blank cartriges, of course). An oversight of this was that it was a training excercise, and the SAS have actually done this in real life in the Iranian hostage rescue - where two of the hotage-takers were talked down, but were still shot despite wielding no weapons.