It's overnight, so you have to book a berth, that's how they get you.
It's 90 quid or so in juice to Dover and back for us, and probably 30 quid to Hull and back. So if you knock the 60 quid saving off the £114 each your into £54 return for the ferry in reality, not sure what Dover to France is any more but the cheapest I ever got it was £35 return, so all in all it still makes sense.
Plus avoid the soul destroying ride home from Dover. It's not so bad on the way out cos' your going on your holidays and are all up for it.