These are the only MUST have documents, which you are well advised to have with you when riding your bike:
Insurance certificate
V5 registration document
Driving licence, both parts
Passport
It says you should have a GB sticker or GB type number plate. Up to you.
Good ideas are:
Spare set of keys (not kept locked in pannier)
Basic map
Mobile phone, activated for foreign usuage, voice and text
Charger for phone, preferably with a plug that fits a French socket. A basic adaptor will do.
Credit / debit card
Some local cash, euro denominated
Dark visor / sunglasses
Waterproofs
Sense of humour
Basic security, disc lock or similar
Breakdown recovery insurance
E111 card
Personal Accident / Travel insurance