Joseanne- just depends. If they have space, you can travel earlier, sometimes for free, sometimes for an extra fee payable there and then. Same for later. But if they are busy and trains full, then no.
For us, the Tunnel or Dover is much shorter and faster, down M11, then on to M20. And then straight down to Dijon. Would make absolutely no sense to go from anywhere else, have a long crossing and a much longer journey. And I would never ever put my dogs through having to stay in the car under decks- in case of delay or bad weather. I have had a few bad crossings on ferry in the past and many hours delay due to unable to dock due to really bad weather- the thought of a dog being stuck in the car with the ferry rollling and the sounds of chains rattling, etc, doesn't bear thinking about.
30 mins in the car, with us- by Tunnel, is the only way.