We took our guns on the ferry to france last year. You will need a european firearms passport from your local firearms issuing office (its free if you e-mail or ring them) and a current shotgun cert. You also should let the customs at the port you are travelling from know that you are travelling with firearms well in advance and you will probably need to e-mail them copies of shotgun cert and EFP. On the journey they will give you a special place on the ferry and you will be checked by customs at the other end. Its all a bit of a number and we know people who just go through without informing customs but not sure what would happen if you do get a stop and search.