When I need or want to give advice on hitchhiking from the UK to mainland Europe or vice versa I usually refer to this page on Digihitch:
Hitchhiking Into and Out Of The UK http://www.digihitch.com/europe27.html
It tells you that it is better to hitchhike to Europe or to the UK on motorway services: Maidstone in UK and Jabekke in Belgium.
However, these pages on the growing Hitchwiki resource mention Calais and Dover again as options to hitchhike:
That's quite confusing to me. So, what is better? To hitch from the services or on the harbours. My guess the first. Would the Hitchwiki need to be edited on this?