The Control of Dogs Order 1992 states that any dog in a public place must wear a collar with the owner's name and address (including postcode) engraved or written on it, or engraved on a tag.
The owner's telephone number is optional. The owner's address is mandatory.
The owner can be fined up to £5,000 if their dog does not comply.
Note, the dog's name does not need to be on the tag (and isn't recommended).