Read the AA pdf and disregard most of the other advice on what to take. You do NOT have to carry a spare bulb kit, or a fire extinguisher, or a first aid kit - it's all just recommendation.
You will also need your licence, your GB sticker, and your V5 as already mentioned though.
Remember that speed limits vary in wet/dry conditions.
On some rural roads you have right of way if you are coming on to a roundabout or out of a side road (this is called "priorite a droite" and is normally clearly indicated - we did not come across this very often)
Petrol is cheapest at Intermarche and SuperU supermarkets which are all signposted (why do we have nothing like this in England?)
Roundabouts are EVERYWHERE and lane discipline on them is horrendous
As you enter a town and pass a sign like this
http://www.controleradar.org/data/speed_02.gif the speed limit automatically becomes 50km/h. On the way out of the town you pass a sign like this
http://www.controleradar.org/data/speed_03.gif which means the speed limit goes back to whatever it was before.
Overtaking on A roads can be hard because of the sheer volume of trucks and the arrow straight roads. Without a passenger it can get immensely frustrating trying to see around things.
I did 2100 miles in France last June and in general it was less stressful than driving in England can be.