As others have said, breakdown insurance is well worth it.
Shop around for the best deal but if you're going over a few times it might be worth getting a years cover. Went over four times last year and had to use it twice.
If you're away for any length of time it might be worth getting travel insurance, be careful though not many places cover you if you're riding a bike above 125cc. Do a search on here, there's a company recommended somewhere that's very cheap. Other than that make sure you take your EHIC card (replaced the E111)
If you're going Dover-Calais it might be worth becoming a sponsor on here as you can get deals on the Tunnel.
As for what to carry, documents and a GB sticker are about the only compulsory things, but a set of bulbs and a puncture kit don't take up much room and could get you out of a fix.
I think I'm right in saying you don't have to convert your headlight (as it isn't directional) :dunno:
Be wary of getting fuel on a Sunday or after 8pm any other day, you're ok on the motorway but in towns and villages you'll need a french credit card or chat up a local into exchanging your cash for them filing up on their card.
One of the best bits of advice I've ever had on France though is, get off the motorways! Their A roads are usually empty, nearly as quick and you get to see the country.