It's not flat, bad areas are swiss normandy region, which is named swiss for a reason, it is incredibly hilly.
I have cycled a lot in northern france, plenty of enjoyable cyle rides, if you need excellent cycle path maps for free, try the nearest tourist office on your trip, they provide them free in Normandy written in english.
This is the pdf of what the tourist office will give you
http://www.mancheran...manche-2009.pdf
It shows all the cycle routes in Normandy.
Also you can get some rock bottom prices if you book the ferry from Portsmouth in advance, i nearly always travel with my bike on the ferry.
Ld lines have given me great service, they had a 5 day ticket where you can visit france for 5 days,i missed the ferry on the 5 th day and they gave me a ticket for day 6 for free to get home.
I also wild camp in northern france, it isn't too hard to find a place.Or if you prefer camp sites, they cost 4 euros upwards.
Here was one of my wild camping spots.
View: http://vimeo.com/19073775