We cycled the Norte and Voie Litoral coming back from Santiago. The Confraternity of St James website would have up-to-date guides for cycling and albergues.
The Voie Litoral is a real treat - mostly level and through pine forests, lakes and endless white beaches, it is expensive for food/drinks/camping tho'. But it is beautiful and has some crazy narrow concrete paths and ants the size of terriers! We wild-camped.
There are plenty of albergues on the Norte - which is great after cycling all the hills...
We used the Michelin Blue maps (1:100,000) which were brilliant, couldn't find better than 1:200,000 in Spain but there may be now.
It is very varied terrain - we found hybrid bikes did well - although my husband broke a few spokes... We were highly laden..
The Sustrans website would be good for the UK bit - there are some good routes through South Wales, and all along the Thames valley - it depends where you intend to cross the water...?
It sounds like a fascinating route,make sure you post here on what happens, I wish you well and Buen Camino and taith ddiogel!
