In reply to Šljiva:
With DFDS, I think it's standard for them to allow you onto a ferry two hours either side of the one you're are booked onto, if there is space. So in theory you could arrive 3.5 hours early and not have to hang about. A couple of years ago, on the return journey, I arrived at Dunkerque with just over 4 hours to go, and was allowed straight onto a ferry that was about to depart. I think that I was allowed to do this as it was a very quiet crossing, and it's not the norm.
Like 1poundSOCKS, I have turned up in Dover far too early for my ferry, to be directed back out of the terminal and asked to return later to catch the ferry two hours prior to the one I was booked on.
Post edited at 14:00