I took the toll highway from Santander (passes through Bilbao) to San Sebastian as we were in a hurry. Fast, good shape, but expensive (it was approx.
$7 in tolls each way and is apparently the most expensive highway in Spain per kilometer).
In San Sebastian, I parked in an underground lot close to the Hotel Maria Cristina. I didn't want to leave the rental car on the street. It was about $6 for the day.
Driving in Spain was pretty easy and even Madrid was not a whole bunch worse than, say, Boston. Since you are from England, careful about drifting over to that "wrong" side of the road, though.