London to bilbao....any advice?

Buy or borrow a guidebook (if you read French the Guide Routard guides are usually the most comprehensive). Spend some time browsing the regional and departemental websites and then when you've got a list of interesting places that are vaguely on your route, join the dots.

If you are camping you may also find it worth having an idea of where the campsites are - I mark them up on a map. Guides like alanrogers.com and the ACSI eurocampings website aren't comprehensive but they make it very easy to research places to stay.

Michelin maps highlight scenic routes in green - I have always found these recommendations reliable. (You can look at the maps for free online at viamichelin.com or it might be viamichelin.fr- or both. It's also worth researching the voies vertes - http://www.voiesvertes.com is a good place to start.

I think it's always a good idea to mix things up - no matter how lovely the coastline, riding all the way along the coast would probably get boring - so mix sections on the coast with sections inland.

As far as daily distance is concerned it's a bit of a 'how-long-is-a-piece-of-string?' question. The best thing to do is do a short tour in the UK and get an idea of what distances you are comfortable with. If you are unsure then err on the side of cautious - there's nothing in the book of rules that says that if you are ahead of schedule you can't stay and extra day at a campsite and do a day ride - or change your route to go on a detour. In any case allow some flexibility for breakdowns and stuff going wrong.

As far as your proposed route is concerned: I can see the appeal of a sea cruise back, but its quite an expensive option. With the big exception of the TGV services to Avignon and Marseille etc, it is possible and fairly straightforward to take your bike on the train in France: for example on the TGV services from Bordeaux and the sleeper services from Hendaye.