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?
Free bus from Dover to London
I just thought it would be useful for someone: I took the ferry from Calais (15:25, 19 euros) to Dover, and there was an Eurolines bus to London. I think you are supposed to pay for it after you arrive in London, but it was really easy to escape paying.