As the regulations state that, to bring dogs back into the UK, they must have been treated by a vet not less than 24 hours nor more than 48 hours prior to the time of entry, you don't necessarily need a vet in the port of embarkation.
Dependant upon the time of your ferry booking and where you are returning from, it can be much more convenient to visit a vet in the place you stop before your run to the coast. That way, as 'Val A' has said, you don't need to kill quite as much time hanging around the port.
We have used both systems and they can work equally well. When not wanting to hang around the port we used a vet in Bourges but normaly use a vet at Boulogne. We visit him late morning then cover the quarter mile to a nearby supermarket to buy wine etc., have a pleasant evening in the van and leave for the tunnel after a leisurely breakfast the following morning.
You pays your money and you takes your pick as they say.
Hope the ideas help.
Olly.
PS - If you aren't already aware of their existence, acquaint yourself with the dangers faced by the dog and caused by processionary caterpillars ( best bet to Google them ). Fortunately, they shouldn't be too active when you are there but it's far better to be safe than sorry. Here's an explanation I just found on one of the many results produced by Google.
"The caterpillars have a bittersweet smell and both cats and dogs will attempt to eat them. Unfortunately this will almost certainly be fatal. One, if eaten, can be deadly for a cat and three or four will kill a medium-sized dog. The reaction to the poison also causes necrosis of the animal's tongue and by the time the owner notices the problem it is usually too late for veterinary treatment to do anything but ease the animal's suffering".