frog_ATC has summed everything up nicely.
Might I add that both Berck and Eu Mers are seriously PPR fields. I have been to both, on separate trips, and only because the weather demanded landing. I was welcomed with sort-of "open arms", but under their sufferance (they could see the weather was bad).
St Valery is very welcoming (I know Madame le President well...long story), but the airfield is worth a visit due to the exceptional welcome. It's way from the nice harbour village, however.
Abbeville and Dieppe have les douanes with 24 Hr PNR or less, check the Frenck AIP for current details, as frog_ATC suggests. Both are very welcoming airfields. I use them in preference to Le Touquet and others.
Mak, you are correct in regard to obtaining French customs clearance on arrival in France (but see e.g Abbeville & Dieppe above, sometimes only 4 hr PNR is required).
Bon vol