Simon, first of all (and meaning no disrespect to CT) you may find a wider audience and more answers on the Cycling UK and CycleChat fora - both have Touring sections.

 Having said that what a fantastic trip to be contemplating!

First of all, I think you'd want to check that you can bring your bike to Le Havre.

I've done a lot of the Veolodyssey/EV1 through France. I believe northern France has taken a battering this winter so that part of the route may be tricky. I've read comments from some that it was boring but I loved my time there. I like the sea!

The east coast of Spain is probably not a good idea in the summer season. Packed! Expensive! However, a little inland can be more interesting and bike friendly - so long as you like mountains. If planning to follow the EV route be aware that some sections are not as developed as others. Be wary of not following CT's suggestions! :-)

An alternative to get you to Murcia would be the Camino Del Cid.  Starting near Burgos all the way to Valencia through always varying country. Road and off-road options depending on taste. Just a whole lot of hills! (Mind you, they'e everywhere in Spain!)

If you're not aware, CrazyGuyOnABike is a great place for bike journals. Search by location (or on a map) and read/see where people have gone before.

However, you've given us very little info to work with. No idea of when (Spain gets hot in the summer!), how (loaded, camping, credit card), how long (100 mile days? 30?) or if solo, pair, group. Those things can be kind of important! 

Good luck!