Hello - we did a similar trip in the summer, ending in Portugal, in an Arrow. France is a doddle and you can either do it low level, or get up to FL65 and follow the Class E 'low level' routes. We did EGHH - LFRS (stop overnight), LFRS - A25 - BMC - B19 - ENSAC - MAGEC - LESO. This was all at FL85, which keeps you well clear of all the low level military activity and the French controllers (who are excllent) will pass you seamlessly from one sector to the next and get you cleared through all the Class D (at least in our experience). The approach into LESO is fabulous!
Another option is to route as high/low as you like down the West coast of France, however beware of the very large danger areas. Our inital plan was favouring routing through these, firstly for the most direct routing and secondly for the coastal scenery. On the way down we were unable to get a crossing, although some will tell you that you can simply route through at your own risk. We were given very positive instructions from ATC to change track and had the inland route ready to go in case this happened. We had checked the activity before departing Nantes and were even told they were cold... On the return, we telephoned Brest Info before departure, checked status, if we could get a crossing, who to call etc. and had no problem coming this way, but i was on a weekend. Brest Info very helpful - can't find their number right now, but I recall it wasn't too difficult to find.
You might want to check out handling etc at Bordeaux. My research suggested they were a bit more LHR like than other French regional airfields. Nantes were a pleasure and not very expensive.
Feel free to PM me and I'll share what I can.