Unless you have a "dangerous dog" as specified by the Dangerous Dogs Act then a dog does not have to be on a lead.

It’s against the law to let a dog be dangerously out of control anywhere, such as:
* in a public place
* in a private place, eg a neighbour’s house or garden
* in the owner’s home
The law applies to all dogs.

Your dog is considered dangerously out of control if it:
* injures someone
* makes someone worried that it might injure them

A court could also decide that your dog is dangerously out of control if either of the following apply:
* it attacks someone’s animal
* the owner of an animal thinks they could be injured if they tried to stop your dog attacking their animal