You also might want to consider the AVGAS price (I do not have the most recent numbers at hand, but I rememember from 2011 something like, compared to France, almost 0.5 Euro more per litre!!!!!
).
You did not indicate the type of A/C for the planned trip which could influence
the choice. But basically I would consider Nantes (LFRS), La Rochelle (LFBH) and, indeed, Biarritz (LFBZ) as already pointed out from which .
Note that close to the major airport La Rochelle is the 1000m long Grass RWY on the Island of Oleron (LFBH), no fees and AVGAS availablable from Total exploited by the airclub.
As for the return trip you should ask the question at home, since the Spanish are not concerned (you get their customs clearance in the LESO office for the outbound flight). But my answer is clearly YES.
Have a nice flt!