
The dog will have to have it's blood tested at least a month after it's first Rabies shot, then if the titre levels are confirmed by the participating labs, then after six months from the blood test the dog may enter the UK.
As yet there aren't any approved routes, and the dog would have to go into quarantine for a few days until paperwork cleared.
In practice if it is a puppy you are thinking of importing, it could not come to UK until at least 10 months old. It couldn't have it's Rabies shots until 12 weeks, blood checked a month later, and able to enter UK six months after that!
http://www.defra.gov.uk/animalh/quarantine/home.html