The only difference between Dover to Calais or Dover to Dunkerque is the length of time you will sit on the ferry, the Dover Dunkerque Ferry would take an hour of your route, however you would lose that sitting on a ferry.
Where's the Dover, Calais route would be the fastest crossing, when you come out of the ferry terminal get onto the E40/A16 heading for Dunkerque keep on this motorway going past Dunkerque, you will then pass through the border crossing between France and Belgium, this is where you'll get the town of Veurne get off at the junction here onto the N8 Ieperse Steenweg follow this road until you get to the town of Ypres/Ieper.
Just after the border crossing you will hit the town of Adinkerke N34 this is called to the British ( fag Allley) well you will get cigarettes at a cheaper price if you do smoke make sure you shop around in the area as you can get some good deals have been told. ( says Roy the non-smoker).
Right along the coastline you will be travelling has a lot of historical importance to the British Army, from the siege of Calais in the Tudor periods, to the retreat and defence of Calais and Dunkerque of the British Expeditionary Force in 1940, so if you're not in a rush there are lots to see in the area you will be travelling along.
Also remember a lot of the towns you may be looking for have changed names from what the British called them in the First World War and what the local road signs call them now so bear that in mind when you are travelling around the countryside of Ieper.
Enjoy your holiday
