When you stop for fuel or anything else, consiously think about which side of the road to ride on before pulling off. It's messed with my head many times. I know someone who was killed riding up the wrong side of a slip road in Germany.
In France, a lot of fuel stations are unmanned and you can't use them unless you have a French credit card. Don't bother stopping at one of those.
In France the cars pull over to let you past. When you get off the train / ferry at Dover, don't expect the same courtesy from the Brits.
On toll roads you will have to buy a ticket - keep change in your tank bag or somewhere accessable. Oh, and don't drop the ticket or they charge you the toll for the full length of the road, however long that may be.
Take the Eurotunnel, much quicker and easier than the ferry, no worrying about strapping down you bike cos you just put it on the side stand and stay with it.
Oh yeah - on duel carrigeways in Belgium / Netherlands, cars / lorries will pull into the overtaking lane when they want to overtake regardless of whether there is someone coming. If you're already in that lane the etiquette is to slow down to let them pull out. So if you are blatting down the overtaking lane, expect someone to pull out in front of you and have your hand on the brake ready.