I'm at present arguing with Brittany Ferries over what exactly we get for our money. We have 2 dogs and when we went through Portsmouth just before Christmas it cost us about £80 return. At Portsmouth the dogs were not scanned, there are no special loading arrangements for cars carrying dogs and the dogs have to stay in the car at all times. In short, we get nothing for our money on the UK-France leg. The dogs are not even allowed in the terminal building.
France to the UK is slightly different in that they are scanned and various forms are filled in, the dogs are allowed in the terminal building but there is no special consideration re boarding etc. I consider the charges made by the various companies to be excessive and out of all proportion to the service received - UNLESS, they have to make some sort of payment to HMG (which I don't know about).
By the way, probably telling you something you know already, but don't forget to get your dog inspected by a vet at least 24 hrs and no more than 48 hours before you check in for the return crossing. Also make sure that the vet signs the passport in all 3 places, sometimes they forget to sign the bit that says the animal is fit to travel and this can cause all sorts of problems.