I don't know about the flights, but you may not have considered the ferries. We returned from Santander to Portsmouth last October on the Brittany ferry - 24 hours to relax and adjust slowly. We had papers and books to read, stocked up on picnic food in a supermarket in Santander, and enjoyed interesting discussions with fellow passengers. I got quite moved sailing past the Isle of Wight in the mist and thinking about family stories of sailing to and from foreign parts! We liked it so much we are travelling that way out and back to continue our camino del norte this April. Obviously you still have to get home from Portsmouth, but there are coaches and trains to London and onwards. (That reminds me, I must ask my sister near Portsmouth if we can leave our car with her!)
I just looked up prices for June - Bilbao to Portsmouth (P&O) is £145 including a cabin, which you seem to have to have. Santander to Portsmouth (Brittany ferries) is £52 with a reclining seat. I expect with a cabin it will be much the same as P&O.
Anyway, the ferry might be an option if you can't get a flight when you need one.
Buen camino