Regarding taking your boat to France, have a read of this thread: PaperWork required for crossing the Channel.
. It will tell you what documentation you need to take with you.
Last year we holidayed in Morbihan and very much wished we had our boat with us. The Golfe de Morbihan (a huge natural harbour with over 300 islands) would be great to explore (although I've heard it has some very shallow parts so caution and charts are needed!) and if you go outside the Golfe, the Atlantic coast is beautiful. A trip over to one of the islands such as Belle Ile is a must. We stayed on the island for three days and it was lovely.
The northern coast of France is also on our 'must do' list, especially Northern Brittany with all the little inlets (the 'Abers' I think they're known as) and the area around the Crozon peninsula.
We have considered taking our boat to France but have not yet done so. If you make it, do let us know how you get on. One thing we did notice was that many coastal places had good slipways, often free of charge and often with good parking nearby.
Regarding raising your battery, I guess it depends how much water you get swilling around inside the console. If you take a lot of water on board then having the battery mounted higher could be handy, I guess. I'm sure much more knowledgeable forum members will give the definitive answer on that one!
I would think, in your position, having a second battery is a nice luxury if you have space for it. Do you leave the boat unattended for long periods of time with a bilge pump wired in? If so, having a second battery could mean the difference between disappointment (because you can't go out as the battery is flat) and happiness! It is also nice to have a stand-by for safety reasons but I would think most RIBs only have one battery. Once again, a more experienced person than me will probably give you a better answer than mine!
P.S. Welcome to the forum!
Edit: looks like Nick already answered the battery question!