In no particular order, you need to think about:
- Paperwork for yourself and the airplane, like POH, W&B, CofA, Radio Certificate, Passport, PPL. If your microlight flies on a permit instead of a CofA (don't know how this works) you need a one-time permit from the French.
- Route planning: How far do you want to fly over water, Navaids to use, frequencies.
- To enter/leave mainland Europe (the Schengen area) you need to fly via a customs airport. Verify that customs is available (might not be H24). Consult a flight guide for France.
- Read the "general" area of the French AIP, or the introduction in a flight guide. Some rules are different. In particular, check the equivalent of "rule 5".
- To enter/leave the UK you may want to fly via a customs airport, but you can also use another airport provided that customs/police/special branch are informed through the General Aviation Declaration. Easiest is to fill this in and fax it to the airfield concerned. They'll know what authorities need to be informed.
- ICAO Flightplan will be required and you need to close it yourselves unless you fly to a controlled airport (with a tower).
- Safety aids required and/or a good idea: Life vests? ELT?
- Might want to find out how to get excise duty back on fuel uplifted for the flight. Contact HM Customs & Excise.
- French pilots flying in France, in French designed and built aircraft (eg. Robins) do not see the need to communicate in English, strangely enough. Make a crib sheet of the most important circuit calls in French. Also some airfields have nobody on the ground, so you might need to talk to "traffic".
- Circuit joins may be different.