Although Community Speedwatch and the police both carry out speeding detection, what is then done with this data is different because of the rights and responsibilities afforded to them. Community Speedwatch programs are carried out by local volunteers, and they have no authority to enforce the law, hence a letter is sent to the registered keeper of the vehicle in the event that a speeding occurrence is detected. The police of course do have the authority to enforce the law, but this is also with a degree of discretion as the National Police Chief's Council recommends that constabularies give a leeway of 10% + 2mph, but it is up to each constabulary how this guidance is applied if at all.

You can read more on speeding and the application of the law here: www.honestjohn.co.uk/advice/driving-advice/speedin.../