You can't put airfields into an ICAO flight plan e.g.
DCT LFQB DCT
It works in the UK because nobody really looks at anything in a VFR flight plan.
The route I last used was
LFMD STP MTL MOU NEV MOTAL KOVAK LGL DVL SITET (UK)
SITET was the UK/French boundary and one needs the EET to it in the flight plan.
This route avoided the military airspace at FL085/095 (if I recall correctly, it does pass over one of the nuclear power station TRAs but they don't go that high) but nowadays I would try the "airway" routes first. I don't have the SIA chart handy though...
What I do remember is that I spent a lot of time in the hotel the night before with the SIA charts and the airspace booklet which comes with them, working out this route. A lower level route was very very hard, without relying on military airspace being either inactive (despite being officially active at the time) or a transit permitted. I might be excessively pessimistic on this topic and I think French pilots don't worry about it half as much... especially if they know their local mil unit and fly just locally as most probably do. Weekend flying is also much easier in France generally, with most mil stuff inactive.