Thanks for the kind comments.
Unfortunately, if we were to spread outside the UK then that would involve a lot of extra paperwork with the charity commission. We are helping to set up a similar organisation in the Republic of Ireland and would happily help you set up a similar organisation in France.
To begin with, the easiest way is to just set up a website to advertise the tandem(s). Once you start getting donations, then a charity is the way to go. In the Uk we were allowed to be a "small charity" which meant less paperwork and only had to Register when income reached £5000. I don't know what the rules are in France, but maybe similar.
I look forward to helping you set it up, if you would like to do that. It is very rewarding.
Alex