At that time of the year, the Channel could be a mill pond. Or there could be a Force 9 gale whipping through.
DFDS are the cheapest guys around. £100 sounds dear for them. Are you going at a particularly difficult day, perhaps at school half term (France and/or England) or Bank Holiday?
DFDS go from both Dunkerque and Calais to Dover; there's only about 34 km or something like that between the two departure ports. Also from Calais you have the option of P and O, the big boys on the run.
I personally never take farries for that crossing any more. I use Eurotunnel which is a drive on and stay in the car train in the tunnel between Coquelles adjacent to Calais and Folkestone, 20 miles or so inland from Dover. It is very fast (34 minutes for the underwater bit), at least 2 or 3 each direction every hour around the clock, reasonable prices around £100 return with far advance purchase, and all you do is drive on, park in a row, switch off the engine, first gear and handbrake on, and chill for the short smooth journey with no weather concerns. When you get to the other side watch for the brake lights ahead of you reflected off the ceiling, switch on and drive off, with the culture shock of you and your steering wheel are now on the wrong side of the car and passing, particularly large trucks, of which there are many, becomes an adventure. Also, if your car speedo is in km/h unless you have a button to change it you will have to convert to mph. 70 mph (speed limit on most motorways) is 112 kph, much slower than in France where you will have been doing 130.
Check your insurance carefully. Leases are different than rental but check and double check. Both the tunnel and/or the boats may be prohibited. Have the right papers. Declare your plans to the leasing company.
For a North American an IDP is mandatory in France unless you pay a lawyer to translate and notarise your licenses. Cheaper and easier to get IDPs.
