Nigelle,
I assume that you can purchase bags of potting soil in France. Any one of the available varieties should do the job for you. Read the labels carefully. You may find that vermiculite has been added to some of the potting soil mixes. But, vermiculite is not necessary for good germination of seedlings. Perlite can be substituted and is often found in the bagged mixes. Perlite is the small white plastic 'beads' that you have probably noticed in potted plants. If you are composting it would be wise to work some into your soil mix, also. Are you reading the 'Winter Sowing Forum'? All sorts of good tips and ideas to be found there.

VERMICULITE & PERLITE ARE AMERICAN TERMS. DO YOU HAVE ACCESS TO ANY LIGHTWEIGHT, FINELY GROUND, PERHAPS EXPANDED VOLCANO PUMICE ROCK IN FRANCE? IT MIGHT MAKE AN OK SUBSTITUTE FOR YOUR GARDEN...TWO BOOKS BY USA AUTHORS MIGHT ALSO HELP W/IDEAS-- "WINTER GARDENING IN THE MARITIME PACIFIC NORTHWEST" BY BINDA COLEBROOK & "FOUR SEASON HARVEST" BY ELLIOT COLEMAN...GOOD GARDENING TO YOU.K