I moved from Switzerland to the UK with two cats, by road. We took the train - you can only do this if you're in a vehicle, though!
It was straightforward at customs for the cats. I took them into the building for animal checks, they checked their pet passports and their chips. One of the cats was so terrified that he was hunched over and stiff, they asked if he was drugged so I had to show them his terrified face. That was it - very straightforward, no issues. As long as your PETS paperwork is in order, then you're fine.
One thing, though. If your vet makes a mistake in the pet passport, do not let him just scratch it out and continue on the same page as they may make you have the animal re-treated for fleas/ticks and wait 24 hours before entering the UK.
I was told as I was planning the move from UK people who moved to Switzerland that the train is far better for animals than the ferry. Animals tolerate the train much better (you're in there for about 20 minutes to a half hour), the ferry rocks them a bit too much and with the train you can stay in the car with them, but you can't stay in the car on the ferry.