I haven't done this personally, but I know a man who did. (Actually, I think he started from Dieppe.) The main problems he encountered were:
France has roughly the same population as the UK, but is twice the size. This means that towns and villages are much more spread out with long walks in between.
Rural depopulation (which, judging from a multilingual discussion in Los Arcos a few years ago seems common throughout the Western world) means that small towns and villages are mainly occupied by elderly people. The groceries, cafes and small hotels which were a feature of every French provincial town 50 years ago have mainly gone, to be replaced by, if you're lucky, a branch of Intermarche on the bypass. The scarcity of food and accommodation meant that my friend had to wild camp almost the whole way to Bordeaux.
Northern France is relatively flat, which makes for easy but tedious walking.
On the plus side, France has a very well marked
network of footpaths (Grandes Randonnées) maintained by the French hiking association,
La FFRandonnée. Coincidentally, they are organising a great hike to Paris this year in honour of the Olympics.
Northern France has many charming small towns and some wonderful examples of Gothic architecture in its huge mediaeval cathedrals such as Amiens, Lens, Beauvais etc.
In my opinion, with apologies to Spanish readers, the food is better in France, but don't tell anyone that I said so.