
as others have said, it's very straightforward, I went to Germany for a pup in January...
your pup has to be 3 months old, it has to be microchipped and then can be rabies vaccinated. The vet fills in the pet passport which you need when you come into the UK. after 21 days your pup can travel - but must be wormed by the vet (and the passport completed) and then you have to wait 24hours and have 5 days to return to the UK. The person you're buying from should sort all this. Pup will be close to 4 months of age.
I went by ferry, but it's similar on the chunnel - you stop at customs and they scan your pup and check the pet passport, you get on the train and come into UK - where they will look at the pet passport and may recheck the microchip.