There are ferry routes from the South coast of England to North Coast of Spain, having studied all the potential routes I think it would make sense to attempt cycling in Spain next year… as it a new country and looking at the maps there is some cycle infrastructure over there (official cycle routes, off road paths, pilgrimage routes, and canals etc).
I think it would be a great experience cycling in a new country, my only concern is it’s the second most mountainous country in Europe according to google. And towns are far in between and not very built up (except along the coast – however coastal routes generally are main roads, that have lots of traffic).
Information about trains back from Santiago asked here; http://community.raileurope.com/raileurope/topics/bicycles-on-trains-in-spain-q7mblqnjbm3vy?rfm=1
Video of route from Fromista, where route in Spain changes direction from south to east (I.e. where I join the Camino) – 8:49 on this vid https://youtu.be/uZwZdBb_KMs?t=529
Edit: I read a comment on a Facebook post today (28/05/2019) about cycling in Northern Spain – Someone said: “I’ve known many [cyclists] head for northern spain only to end up getting the train out of the there as they could not cope. “, saying it’s “hilly as feck in parts”. I don’t mind hills, and have cycled many hilly tours. Notably my bicycle tour along the Brittany coast. If I were to lower the mileage each day, then I believe it’s possible to cycle this – but looking at terrain maps and elevation data I don’t deny this route would be a big challenge.
I have also read stuff about people stressing the importance of carrying a lot of water in rural Spain – due to the hot weather and how far towns are in between inland.