Legally, all dogs must wear a collar and ID tag when out in public; this applies whether the dog is on a lead or not. The ID tag must detail the owner’s name and address, including postcode. It is also recommended that the ID tag shows a contact phone number.
Since 2016, it is also a legal requirement across the UK to have your dog microchipped by the time they are eight weeks old. Your contact details must be held on one of the government standard registers, and be kept up-to-date.