According to the Dangerous Dogs Act 1991:
1. The following dogs, and also cross-bred pit-bulls, are dangerous dogs under the meaning of the Act:
Pit Bull Terrier
Japanese Tosa
Dogo Argentino
Fila Braziliero
It is illegal to possess such a dog without a certificate of exemption, which is granted once the dog is neutered, insured, and has a transponder implant. These dogs cannot be "in a public place without being muzzled and ... being securely kept on a lead by a person who is not less than 16 years old."
A public place even includes locked inside a car which is itself in a public place.
2. If a dog is dangerously out of control in a public place, or a private place where it is not permitted to be, the owner; and if different, the person for the time being in charge of the dog, is guilty of an offence.
"A dog shall be regarded as dangerously out of control on any occasion on which there are grounds for reasonable apprehension that it will injure any person, whether or not it actually does so."