when driving on the mainland there are afew things that you must carry by law. these include
full bulb replacement kit
first aid kit
and a couple of other things but ive been on the wine so cant remember exactly off the top of my head. If i were you i would take your v5/ license/ mot/ insurance stuff just incase. but from my experiencer french and belgium police tend not to bother with english plated cars.
EDIT: if your number plate doesnt have GB on it youll need a GB sticker aswel. You can buy the pack to make your car legal on most ferries but it will cost you a small fortune. Think it cost me about
ex white Phase 1 Gti6, killed at cadwell park in November 07