It's a lot easier for dogs and cats now they've introduced the UK Pet Passport system. But rodents are outside the scheme. Saying that, a friend and I took a dog from Luton to Holland and back via Eurotunnel and the tunnel people insist on charging £50 for the dog's "fare". It's just sitting in the car!
I've exported hamsters to Sweden via the ferry but never tried it the other way round. I've only imported from EU (Holland, Germany) using Eurotunnel. I know some hamster breeders in Sweden/Holland/Germany so will check if anyone's exported to the UK recently and if so, what the security procedure is. Ideally if I can get him into Paris avoiding quarantine (or breaking the law), we can just hop on a train to Calais, take the ferry (as I assume they don't security check hand luggage?) and then get the train from Dover to London (or make my poor father drive down from Lancashire in my car - that's stored at his while I'm in HK) and pick us up.