Are you starting from St Jean Pied du Port or from somewhere else? One tip I can offer if it is St Jean Pied du Port, you can get water there, water at the first refuge and water 11kms after that from a 'hole in the wall' type fountain. The next place is Roncevalles.
I've run out of ideas for the UK, I live on the coast so the idea of walking along the coast doesn't appeal to me at all. The only walk I can think of that might turn me on again is the Pennine Way, a good enough distance and one which at least makes a demand on one.
I'd like to go to France again, so many routes in my head, GR5, GR10 from the Med, half the GR10, half the GR11, It gives me a headache! I'd love to hike the GR20 but its just the faff and cost of getting their and back that stops me, for the prices I've seen I could hike two French mountain routes!