I've done that trip 4 times now in a Bongo and going back to the Alps again in September.
DFDS Dover to Dunkirk is the way to go, book directly with them on line and it can be as low as £28 each way (off peak, night crossing, advance booking). Advantage is driving on quiet UK roads in the evening, and passing through the busy part of France / Belgium in the early hours before rush hour.
Best route (No Tolls !!!) is through Belgium (skirting Brussels) to Luxembourg, then down through Metz, Nancy towards Besancon and your final destination. Roads are all excellent and perfect for bowling along in a Bongo at 60-65mph. Use a good Sat-Nav (preferably with fuel station POI's) and it is an easy, stress free drive
Driving standards out there are excellent (better than UK) as nobody hogs the outside lanes - but remember to pull in as soon as you overtake lorries etc (as they all do) or they will drive impatiently up your @rse ! There are loads of 'Aire de ....' rest areas with picnic tables to stop at en route - often in nice locations.
Don't fill up before the ferry as there is a cheap(ish) petrol station near the Dunkirk terminal. Aim to arrive almost empty at Luxembourg and fill tank to the brim as it is MUCH cheaper there. Fuel up at supermarkets in France as it is at least 10-20c less than motorway service stations.
The Bongo made light work of driving 4000 feet straight up the Alps several times a day, with up to 6 adults and paragliding equipment on board. Just remember to leave the engine running for a bit to cool off after any steep ascents.
PS - if you haven't yet booked, I am just about to buy another "multitrip package" from DFDS which works out slightly cheaper and has the advantage that tickets are open and can be used (by anyone) on any off peak crossing - so no surcharges if you miss your ferry or want to change dates .....
PM me if you want one of these tickets, which can be used any time before end of 2012. (or if anyone else is interested in going).
![Very Happy :D](./images/smilies/icon_biggrin.gif)