The ship in question is the French amphibious assault ship "Mistral" which I posted some photos of on the "Royal Navy" thread the other day.
http://www.doverforum.com/letters/viewtopic.php?id=4142
The "Ark Royal" has a totally different profile with two streamlined funnels and a long low island structure, together with the ski ramp in the bows. Her two sister ships are HMS "Illustrious" and HMS "Invincible." The "Illustrious" has visited Dover in the past, berthing on the Admiralty Pier, and is seen here passing Dover five years ago with a Harrier posing on the ski ramp.
More photos of the "Mistral" off Calais on my fotopic site at:
http://shipsintheportofdover.fotopic.net/c1839532.html