If you live within striking distance of Dover then making it there for an early crossing is feasible, if not then commiting the first day to getting out of UK and spending the night in northern France makes sense. The +1hr difference on top of the time spent crossing the channel dents the early start some what.
The choice between train or ferry is a personal one, based on whether you treat it as an opportunity for a break on board the ferry, or if you just want to get it out of the way as quick as possible on the train. Plus cost and availability come into the equation as well.
I have done Essex to Bad Kissingen in Germany (a little east of Frankfurt) twice, including the channel crossing (train) but ignoring the time difference, the shortest time was 11.5hrs, the longest was 13.5hrs. This is a long day and was only done to get from point A to point B, the next day was not spent in the saddle. It is not best practise to cover these distances day after day, after all you are on holiday! The journey is all part of the adventure, and the holiday starts as soon as you leave the driveway.
An early start from an overnight stop in northern France will stand you in a good position to cover ground on your first day as group, (there are some nice campsites perfectly suited for this). Try and keep your daily mileages attanable, factor in a break every 2hrs, its amazing how refreashed you can feel after a 15min break or 20mins sleep. You should also factor in a couple of rest days during the duration of the trip, otherwise gradual exhaustion can set in, and you risk returning from the trip needing a holiday to recover!
The trucks will also appreciate a rest now and then, as don't forget many of them will be fully loaded and working hard in the conditions of a European summer, especially when at altitude. When pushed to the limit is when the trucks can start to show up an issue which would go undetected for years during normal use.
If long days are planned and no wriggle room given in the schedule, any little delay that crops up could see you arriving at camp under the cover of darkness, or constantly chasing the schedule for the duration of the trip.
You guys are right to be disscussing and planning this now, so that you have a smooth and enjoyable trip.
Enjoy!