I moved myself, household contents and two cats from Switzerland to the UK by road. The person who did the move was kind enough to let the cats and me tag along for the ride with the household contents.
We took the train, which was definitely the best option. On the ferry, you cannot stay with your car, so your cats will be alone in the car. From others who have done the journey, apparently the ferry is a much tougher ride for pets because the ride can get very rough down where the cars are stored.
When arriving at the train station, you first go through customs and they'll give you a sticker to put on the car for animals on board. You then follow the signs to check in animals. It is a small building on its own. You take them in, the staff will check their passports and microchips and just make sure all is ok. One of mine was absolutely terrified, beyond words (the poor boy!) and was very stiff and hiding his head. The staff asked if he was drugged, so I needed to pull his head up so they could see him (and his wide-eyed full-pupil look of pure terror!). That was it, the check was complete and they could enter the UK. Back in the van, in line to get on the train and drive on the train. Once the train gets underway, it is about 20 or 30 minutes to the UK. You drive off the train and on your way.
Just make sure that if your vet makes a mistake on their passport that they start a new page and not cross out info and correct it! We nearly needed to stay 24 hours because when I had the flea/tick treatment, my vet put down the wrong date of the treatment and corrected it on the same page. They let me through mostly as a courtesy, I think, after seeing terror-filled cat (and a second cat with a perfect passport from the same vet), and did mention what to do next time if the vet makes a mistake.
This was in March 2010