You could swim to Pas-de-Calais from England if you were so inclined – and had plenty of goose fat rubbed in against the cold. For the less hardy, Pas-de-Calais is still reachable from Kent in a blink: its capital Calais directly faces Dover across the Dover Strait of around 21 miles, and, once you arrive, there’s a white cliff or two around here too. (There are reasons why Dover and Calais are twin cities.)
As it’s so close, many people go camping in Pas-de-Calais from England for the weekend, but we say pffft to that and that you should come for at least a week: the Côte d'Opale/Opal Coast lines the Pas-de-Calais shore between Calais and the old art and history town of Boulogne-Sur-Mer, a distance that we say is walkable in one trip (even with stop offs at Aqualud water park and Le Touquet, aka Paris-by-the-sea).
Like its departmental neighbour Nord and mothership Nord-Pas-de-Calais, this northern France region has many a war memorial: ask at a Pas-de-Calais campsite what’s nearby to see, but take the time if poss to see the giant German World War II bunker La Coupole near Saint-Omer, now the site of a WWII museum and 3D planetarium.