My 2pence worth ...
Some of the stuff you may want to consider is because the local laws say you need them. For example, you need to carry a first aid kit in a lot of Europe, but I've never seen a definition as to what should be in one. Not a bad thing to carry one anyway. Amazon has them for a few pounds. Same with a spare bulb kit. I even carry H7 headlight bulbs for my SX - as much use as a chocolate teapot in reality, but I have them to comply with the rules.https://www.amazon.co.uk/gp/product/B00GOL9ZW8/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1
Warning triangle. Amazon again. A small one. I'm sure there is a definition somewhere of the exact dimensions and mine may not comply, but I have one with me.https://www.amazon.co.uk/gp/product/B07FLGG4M6/ref=ppx_yo_dt_b_asin_title_o02_s01?ie=UTF8&psc=1
Hi-Viz ... I have dayglow panels on my jacket, but I also have one of the excellent Oxford over jackets. If that isn't hi-viz enough ... I'm lost https://www.sportsbikeshop.co.uk/motorcycle_parts/content_prod/244260
Tools - Cheap multiman tool. It might be a one-shot wonder for 15 quid but you only need it to get you out of trouble. Duct tape fixes everything but stupid. It can even fix a leak on your clothing. Puncture repair kit and know how to use it. Cable ties. No need to double up on everything, share the list with your mates. You only need one breaker bar between you for example. Fuses ... Often forgotten. Visor cleaner and a microfibre cloth.
Documents ... Original copies of the V5 and Insurance. Take photos of these with your phone just in case you lose the originals. Same for your passport. So much easier when you can show a photo and explain the original was lost or stolen.
As others have said, a complete set of spare keys and swap them with your mates.
GPS and speed cameras. France does not allow speed camera warnings. I could not comment on having the excellent SAMCAM application running on your phone in your jacket - honest you were listening to music. Caution mobile data charges in some countries.
Inferred speed limits. When you pass a sign with the village/town name on it, it often marks the start of the town speed limit. 70kph at best, most likely 50kph. When you pass the same sign, this time with a line through it, as you leave the town/village, that marks the end of the reduced speed limit. Don't take this as an absolute, there is always going to be somewhere this doesn't apply. There often isn't a large 30mph (50k) sign as we have.
Policemen ... Doesn't matter what country you are in there is always "that" policeman, and if you get him (or her) then it is going to hurt. The sarcasm and rhetorical questions are disrespectful enough in English - something in the training about establishing dominance in the conversation - luckily I don't speak french and so have never understood them.
To that point ... don't have a wallet stuffed with all your cash. It is in the minority, but there have been cases of the fine matching the amount of money available. I'm not suggesting corruption, just if you have 200 euros the fine is 200 euros. If you only have 50 ... you get the idea.
French motorways are excellent for getting high miles done, but cost a small fortune in tolls. I've never found the motorcycle lane that a few people speak of, but then again I prefer the non-motorway routes. About 8-12ks before a toll booth you will often pass under a gantry. If fact any gantry across a motorway will often include a speed camera.
If you do get done the bill arrives in the post a few weeks later. They reduce the speed the camera caught you at to allow for any inaccuracies and then issue the fine. It is only a fine. No points go on your license. There is a discount for paying promptly, which you do via their website. All very efficient.
Turning across double white lines. In the UK we turn right across double white lines. Not so the case in Spain (don't know about the rest of Europe). Often there is a blue square sign with what looks like a figure 4 in it. This is telling you to go to the next junction to and double back. Most of the time this is a roundabout or a slip road that takes you under or over the main road. I've seen policeman sitting in the cafe just waiting to "raise public awareness of the issue".
If you are avoiding France and going the Holland - Germany - Switzerland route to Italy, then check the need for Vinettes - These are stickers you buy at the local shops. Like road tax. You can get them for weeks or months as well as a year. Why we don't have them for overseas visitors ... oh well.
Filtering laws differ from the UK. On Germany autobahns, there is a rule that when they stop car drivers are required to pull to either edge of the road to make space for emergency vehicles. This is such a tempting lane to filter down, but the German police can take a very dim view of it. It is OK to filter if itis road works, just not accidents. How you work out the difference from 5 kilometres back remains a mystery. Often there will be information on the motorway signs telling you about roadworks. If you didn't see it and the traffic slows, proceed with caution or not at all.
Right, I've waffled on for long enough. It sounds like a lot but it isn't difficult. The roads are in better condition than most of ours. The coffee stops are excellent. Sounds like an excellent trip ... Enjoy