Under the Dangerous Dogs Act 1991 it is an offence for a dog to be dangerously out of control in a public place. For the owner to be prosecuted the dog does not need to be in a public place, but simply for the animal to either bite someone or to put someone in fear. In this case contact the police.
The local authority is responsible for picking up and keeping stray dogs. The police only keep stray dogs brought to police stations until the council takes the dogs away. Your local police station has a list of lost, and found dogs.
Certain breeds of fighting dog such as Filo Braziliero, Pitbull Terrier, Japanese Toza, and Dogo Argentino can only be owned by people over 21. These dogs have to be on a lead and muzzled in public and have certificates of exemption.
If you are attacked, or bitten by a dog, seek medical attention, then contact the police. The incident should also be reported to your local authority dog warden.