We do the Tunnel with our two, so can't help with the ferry.
However, as far as vets are concerned, try to contact a few via email before you go, to find out their charges. (Perhaps other pet owners on this website can help if they have used a vet somewhere along your homeward route).
We paid a 'consultation charge' for a vet in Thann (France), to administer the worming tables to our two dogs in October. We had taken our tablets with us, so the vet only had to give the tablest and fill in the paperwork. He didn't charge us 'per dog' - just for the one consultation.
I know some people have been charged 80 Euros for one dog! (That included the tablet!!!). - We paid 30 Euros in total.....but had been told the cost would be 60 Euros by a vet in Nancy! I also know of one person who took their own tablet to a vet in rural France, and the vet administered it and filled in the paperwork free of charge! It is worth checking prices first if possible.