Just to add to what others have said in their replies, don't, unless you have lots of money, buy a breathalyser on the ferry or at a well known UK automotive store. They will rip you off. Buy at a French supermarket, if, indeed you can find any, other than Super U, that are bothering to stock them. It seems like another French white elephant.
Out of interest, has anyone out there actually been asked by the French Police to show that they have got one in the car ?