This is the route along the English Channel (la Manche), from the Breton port of Roscoff to Dunkerque and the Belgian border.
Unlike the Atlantic coast route (which more or less takes a direct course parallel to the sea), this is an intricate, winding route along minor roads, occasional rail-trails, and seaside promenades. As such, its 1300km (800 miles) is significantly longer than the straightforward 800km route would be. But your reward is a constant series of clifftop views, visits to fishing villages and old-style resorts, and historic sites from the Bayeux tapestry to the D-Day landings.
The one place where the route departs significantly from the coast is the Cotentin peninsula (Cherbourg). That said, there’s an optional loop from Carentan to Cherbourg.
The terrain is undulating rather than flat, with countless trips down from the 100m contour to sea level every time a river flows into the sea.
Transport options are of course excellent, with the Channel ferries and many local trains providing connections throughout France and to Britain.