Rural France is a lot more likely to suffer less thefts than most parts of Britain imho, so whatever you use in the UK should be fine for France...I just can't see the issue here? Targeted thefts in urban areas frequently involve a van (probably nicked too) and three or four beefy guys so weight of bike is no prevention against theft. Chaining it to something solid is as decent a deterrent as you're likely to provide, along with other locks made to be a PITA to remove. I usually D lock front and back wheels as well as use a chain, and alarmed disc locks. If they nick one of my bikes secured this way, I've done as much as I could have done