In theory you are supposed to carry a fire extinguisher in Belgium, and a medical kit and warning triangle in France and Germany. Halfords do a kit that has a GB sticker and this stuff, but to be honest I wouldn't worry too much, I very much doubt that anyone would notice.
The one thing you must do is make sure you have nothing that could be mistaken for a speed camera detector. They are very strict on this in France, and if they suspect you have one they will apparently rip your car to bits in order to confiscate it, even if it's a GPS jobby that doesn't work in Europe anyway.
Tim - not much point bothering with travel insurance I would have thought. Fill in an E111 and that takes care of medical care in Europe, and your car insurance probably covers everything else.