So
@FionaB, what's your thinking? Ferry across the channel and cycle through France, or a Ferry to Santander and just cycle in Spain?
I've taken my bike on the Dieppe Ferry, it was a painless experience. And quick through customs. The ferry runs around 10am and 10pm, so you either arrive in Dieppe at 2am in the morning, or you have to cycle through the night for the morning ferry. I did that, left at midnight and stopped at Gatwick for a mid ride coffee and panini from the Costa inside the terminal. Across the channel I just nipped across to Paris, so can't really help you there.
That said, I've found strava routes useful for places I don't know, because it shows you the roads the locals use - so here is what it suggests
https://www.strava.com/routes/7725688