It is the way I go to Camino, and very pleasant it is. From Santander there are regular fast and cheap buses, just a few Euros, (they are comfortable inter-city coaches really) to Bilbao bus station and change there for Pamplona bus station - arrive Santander, arrive Pamplona same day, a few hours later. One can start walking at Pamplona but at Pamplona bus station there are buses to Roncesvalles too and if really keen a taxi from Roncesvalles to St Jean is doable.
Also from Bilbao there is another express that avoids Pamplona and goes direct to Camino at Puente la Reina and then Estrella and on to Logrono.
Brittany Ferries have two departures terminals from the UK; Portsmouth and Plymouth - the ferries sail on different days.
There is a direct ferry to Bilbao but before the pandemic and they shut down you couldn't go as a foot passenger, had to have a vehicle.
The ferry is 24 hours and is quite cheap but you do have to book a cabin as well. It pays to book ahead as the cabins get filled up quickly as you book a whole cabin for yourself and once they are gone they don't take more passengers.
I have always thought that it could be more pleasant for North Americans, Australians, and New Zealanders to arrive UK, visit for a couple of days and lose the jet lag, then take the train down to either Portsmouth or Plymouth and arrive Spain that way. Who doesn't like a pleasant sea cruise???