Phil, Would be good to hitch on to your group, we got lost last year although I think we would know better this time.
I have heard that the French are only concentrating on the roads within about 30K of Calais. They have always had the ability to take the license it hapenned last year to a driver of a subaru caught doing 50 in a 30 who wasnt "humble" when caught, he had to get a friend to drive back and take the car the rest of the way, interestingly the car behind him was a ML Merc and he just got a fine as he was not a young lad and new his manners etc.
We found last year that it was much like the UK (only faster), if you get on a run of cars doing 100ish and the occassional blip to 120 or so then it just seems normal. If you hit a batch of cars doing 80 then its stupid to blast through at 110 or so.
So much of the road is miles from anywhere, they would have to be very well hidden to get you.
Anyhow I am looking forward to it, lets keep in contact and make arrangements nearer the time.
Best Regards Si