Most of the French filling stations will sell you a decent quality, brass cup adapter for around 8 Euros. You are right about the speed cameras, they are popping up everywhere but can be ignored. There was talk about 3 years ago that there would be cross border agreements so drivers of foreign registered cars could be contacted and chased for payment. Unfortunately, the EU instigated Data Protection Act doesn't allow DVLA to pass personal information to another countries authorities so you can get flashed with impunity (same goes for parking tickets too). I once set myself the target of being flashed by every speed camera between Nice and Calais. I succeeded but never heard anything..... Beware of manned speed cameras though. They will stop you and demand an on the spot fine. So if you see a marked car (often something like a Subaru Impreza) lurking at the side of the road, beware, he'll be pointing a speed gun at you. They have their favourite spots of Autoroute where they sit and they also sit just inside the sign to tell you when you enter a village to nab you in the 50kph area.
Interesting you say that - Someone I work with got penalty notices on a uk registered car for non-payment of the m50 toll in Dublin. Not sure how he missed the signs for it as they are somewhat obvious though suspect he may have got confused by the toll for the tunnel to goto the port (which you pay at a barrier, rather than the m50 which is barrierless like the Dartford crossing is now). There may well be different agreements with Ireland and the UK due to the shared land border though i guess.