Anywhere along the route Santander -> RN -> Roscoff?
Across the bottom of France, broadly on the horizontal. Then, up France and then across France on the diagonal. That's a fair old distance to be looking for suggestions over. But, hey-ho let's go.
Suggestions:
(1) If you haven't got one already, get a decent map or two (see sticky). The maps suggested are excellent for plotting just the route you have in mind.
(2) To give everyone an idea of what you are looking at, here is your route hacked out in the dreaded Google maps. First of all using motorways and toll roads (boo, dull, mate)....
http://maps.google.co.uk/maps?saddr...DFFSFvQ-e_afg&oq=roscoff&gl=uk&mra=ls&t=m&z=6
(3) That rather crude hack out looks like about 1,500 miles. Let's use your seven days between the two ferries, so that's about 200 miles a day every day with no days off. Does that suit you? Let's then say two days off, which gives you five days, or 300 miles a day on the days you move. Any good?
(4) Let's then get Google maps to give us it avoiding tolls and motorways (great roads, mate)....
http://maps.google.co.uk/maps?saddr...coff&gl=uk&dirflg=ht&doflg=ptm&mra=ls&t=m&z=6
That's getting a bit more like I imagined.
The mileage is about the same at 1,500 so the days on and off the bike / daily mileages are about the same, the roads (as your maps will show you) are better though. It's roughly what I would do, if I were doing what you are doing.
(5) Now comes the tricky / fun bit and it's your turn. Nobody has the slightest idea where you want to go each day along the line, so guessing where you should stop is very difficult. The easiest way is for you to say to yourself: We want to do no more than 250 miles a day. Then simply run your thumb along the line on the map stopping at 250 miles. Then seeing which town or village you would end up in. It's a safe bet that if you want to stay in an Ibis hotel, you'll need to be finding yourself in a largish town. If you want a Logis hotel, a large or smaller town might well do. If you want no town or just a tiny village then get busy on the web.
(6) The horizontal line you could maybe pull up a bit, perhaps? It would bring in some good roads and go deeper into Provence, to then go back down on the diagonal to say, Grasse. But a lot of that will depend on your thoughts on point 5. What you might though think about is seeing the Camargue, as you are not far away. Flatter than a witch's.... but unusual. Nice beach at Saintes-Marie-de-la-Mer, if you fancy a paddle.
(7) You are going with chums, no? I suggest an evening in the pub, maps to hand. Each of you do a separate leg, maybe? It's your holiday after all.
