...I think you can add a trailer to the return ticket, if you get to the booth early, well before sailing time, n pay the extra few pounds for it.
.... incidentally, you can still buy your ticket for the sailing at the booth at the ferry port, provided they've got room on the boat....
....and, from experience, if you pay for the ticket at booth in good auld fashioned cash, this will result in you and your passengers and the car being completely searched, and a bit of an interrogation by police n customs, and you go from the front of the outside queue to the last to get on the boat....