Bring the dog to the pound
Bring the lost dog to your local pound. To find which pound the dog should go to, search the address where the dog was found on this website to see what council the dog was found in. Then check the lost pet section on that council's website. Some vets may be willing to scan the dog for a microchip and try find the owner but they are NOT legally obligated to scan (and many are unable to hold) dogs so call them first before going there.
It is illegal to keep, sell or give away a dog that you have found and if you do so, you may be prosecuted. If you wish to adopt the dog if not reclaimed from the pound, let the pound staff know.