It's not too difficult to do wild camping, once you have found a few spots you'll be spotting places all day long while cycling.
In all i've done around 40 nights wild camping, and as yet haven't used a proper camp site.
I wild camp in France,belgium,england,wales and scotland.
Here are some ideas, find an official cyclepath in france and you'll find a huge amount of camping places free of charge,The cyle paths often go straight through wooded areas and there is no one around after dark.
Camp somewhere where there is little sign of life, don't camp in the centre of town or in really busy areas, you can comfortably camp on the wooded areas right next to main roads out of town.
Use trees as camouflage,my ideal camping spot is in the middle of 2 trees where i can't be seen at any angle, good for rain cover too.
I am going straight to Gibraltar in a few weeks and will be wild camping most, if not all the way.