All excellent advise above! I'd like to add my tuppeny's worth too if i may....
When navigating on N and D roads,largely ignore the road numbers as they firstly are rarely mentioned until you're actually on them and secondly they tend to change as you cross into a different "county",follow instead the signs for the next PLACE on your list(ie:village/town). If when you approach a town or village you don't see the signs directing you to the next place you have in mind,follow signs for "centre ville" and you'll come across the needed signs soon enough....often bypasses don't include destinations to everywhere nearby.
"camping municipal" as previously mentioned are generally excellent and very safe places to leave your tent with gear inside while you go sightseeing,they're more often than not populated by retired Dutch couples who are the nicest people you can hope to meet,speak better English than a lot of Brits i know(!),and will be only too happy to keep a lookout for you when you're gone.
Don't worry about your limited French.To make the effort to speak their language means more to them than getting it hopelessly wrong....and it'll inject humour into the situation which is such a valuable thing in today's world.Old men will come up to you and ask you about your bike,where you've come from,where you've been,etc...even if you don't understand what they say,take time to try and answer as best you can.The pace of life in rural France is totally different to the UK.
Be aware of the speed limits(though the locals don't always).When on the open road it's 90kph,when you pass a village boundry sign (red border on white sign,black writing) that's the start of the 50kph limit unless it says otherwise,and the end of the limit is the same sign in black writing/border with a line through it.Take note when other motorists coming the other way flash you...there's a "gendarme" with a hairdryer nearby.
Roundabouts....hmmmm! If it's even remotelly damp take HUGE care....they're as grippy as Teflon,which is about as grippy as the white paint is in the midle of the road too.But the flip side is that you'll notice the fuel ecconomy of the bike will be better than here in the UK as the road surface is less abrasive over there.
Fuel is much cheaper in the supermarkets than anywhere else although out of hours these can be a pain as they revert to "automat" mode when the attendant goes for lunch/home and as of last year most won't recognise British credit cards.If you get caught out simply wait until a local stops to fill up and ask if you can give them cash to use their card to fill up.I've never been refused yet.
Speedferries will be the cheapest operators but if you want to sail Portsmouth / Le Harvre on a night boat then LD lines are the best bet.I tend to do that as it means i arrive fresh in the morning ready for a full day's ride.
Above all take it easy and ENJOY!

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Just going for a short ride on my bike....