France is a fantastic country for cycle touring; I don't think you could go wrong whether you are in the mountains to the south and east or the flat coasts of the west. So my advice is just go and see if you like it. French summer campsites are expensive in tourist areas but cheap when you are off the beaten track.
Solo cycle touring is my "thing." I love it - but Wowgagger touches on a point above - it can be lonely on campsites if you are the only solo and your language skills are limited. If that concerns you, why not try one of the CTC organised tours? They are highly recommended. Alternatively, look at the French Youth Hostels, they are good, cheap and sociable - assuming that you don't mind sharing a dorm.
I would find it very difficult to meander around on a bike, no matter how nice the weather or how beautiful the country. You might have a mindset (like mine) that always needs a purpose or goal: for example, Rome is barely 1000 miles from the French channel ports - why not cycle to Rome, or Madrid? That's a very, very easy three weeks - then fly back.
These routes/ideas are also on my "to do" list.
1) Ride from home to Santander, ferry back to Plymouth or Portsmouth.
2) Ferry from Harwich to Rotterdam, then ride to Roscoff, ferry back to Plymouth.
3) As above but first ferry to Calais - so it's shorter.
4) Checkout the Ryanair and Easyjet route maps and fly cheaply to absolutely anywhere that's about 1000 miles away - northern Spain, southern France, northern Italy, Denmark, Poland? - then ride home.
5) Go and visit friends/colleagues who have a house somewhere in France.
My last point (sorry for rambling) is that 50 miles a day is probably too short if you are just cycling. On a good day you'll be finished by lunchtime and then what? If you decide to visit cathedrals, or follow canals or paint or something - that's fine - but the afternoons and evenings can be long and boring if you don't.
Pick a target.