Obviously this option was for those rather well to do, as I imagine it wasn't a cheap option.
I suppose in those days it was only the "rather well to do" who would have a car, let alone take it to the continent!
They heyday of Silver City and Channel Air Bridge was before the advent of roll-on/roll-off car ferries on th Channel and taking a car by sea was not that easy. In fact, Silver City were so successful they often lowered their fares year on year in the 1950's.