Check the local laws
There is the new French law about hi-vis, but I am not sure if this applies yet and if your jacket has reflective panels (most do) it may be enough.
Carry a full set of spare lights and fuses, warning triangle, have a hi-vis in case of breakdown (if not already wearing one, reflective panels on your jacket are not enough in this case), first-aid kit, ownership docs (originals), license (original), passport (original), insurance docs (original), afix beam deflectors, make sure your plate declares "GB" and is 100% legal (afix a "GB" sticker to the bike if the plate is not new-style), if you have a radar detector (or something that could be confused with a radar detector) remove it and ride with dipped beam on during the day.
AIUI the above are legally required and you could face penalties (including having your bike seized) if you do not comply.
DO NOT drink ride. Not only is it bloody stupid, but the French limits are stricter and the fines severe.
Afix a "Keep right" sticker to the yoke, and learn about the road use differences; if "priorite a droite" means nothing to you, you'd better do some research!
Make sure your driving insurance covers you in Europe (some don't), make sure you are not going to massively exceed your annual mileage limit (contact your insurer), carry breakdown information (this may be in your insurance, make sure it covers Europe; many don't).
Have the bike serviced before you go, probably fit new tyres too if they will need changed during the trip (all depends on the bike really). Carry some chain lube/oil, gaffer tape, cable ties, puncture repair kit, pressure gauge and make sure your tool-kit is compete (you can carry less if in a group, spreading the weight around).
A phrase book is a good idea, as are up-to-date maps (even if you have GPS, maps don't need batteries).
Make sure your mobile will work, credit-card too. Consider a pre-paid credit-card. Medical insurance (make sure it covers motorcycle use, most do not - ask explicitly and get it in writing; check it provides evacuation back to a UK hospital too). Get your EU health-card (your insurance may require this).
Wear decent gear and take good base layers. High elevations are cold, even in summer.
If in a group, know how to convoy ride and if one person stops for fuel; all stop for fuel. Drink bottled water/juice, not coffee/carbonated-sugar-drinks.
Will your arse cope? Consider something like an "AirHawk" pad; note that gel can get very hot in direct sunshine.