Someone will jump in here and correct me if I'm wrong, but I believe your dog also has to be "chipped" to be eligible for avoiding quarantine. That's the little id chip they implant in the dog's neck to prove his identity (i.e. that he really is the one who had the blood test showing no rabies). You may also need to prove that he has been vaccinated against rabies.

There is a slight catch in getting a dog chipped in the US - and that is that there are two different sorts of chips used. Only one of them is acceptable for entry into the UK. Check with the British Consulate or Embassy about this to make very sure you get your dog the right chip. Most veterinarians in US don't know about the two kinds, nor which one is used in Europe. Just be sure you get the right one. Installing the chip is a pretty easy procedure your vet should be able to do for you.

Coming back will actually be easier than going to the UK. I think you only need to show the vaccination record for the dog and possibly a health certificate that he has been looked over by a vet and is in good health.
Cheers,
Bev