With regards paperwork, BassBoy's list in post 17 above is pretty comprehensive: whilst not all are strictly necessary, the French love of bureaucracy means that you should have all the paperwork available. I keep the originals in a waterproof wallet with laminated copies also available.
Additionally, however, bear in mind that the Channel Islands are outside the EU so that if you are planning to call there, you need to complete and deliver Part 1 of Customs Form C1331 before you leave the UK and follow the reporting procedures on the back of Part 2 upon your return before then delivering Part 2. The form is downloadable from: http://customs.hmrc.gov.uk/channelsP...HMCE_CL_000428
Since part of the reporting procedures involves flying a Q flag upon re-entering UK territorial waters until customs clearance is given, you'll also need a square of yellow material. I don't know how many people actually bother with this: certainly Customs seemed surprised when I bothered and were more interested in what I'd had by way of a rather nice lunch in Alderney than any duty free goods that I might have had.