Explorette, we travel to Spain quite often and use Brittany ferries and as we are club members we get a discount on the fares which guests can use to also get a discount, if and when you decide travel with Brittany ferries you can use the club members discount by applying for Guest discount using the Code No. S0016R in the "guest of member box" discount will be applied at the final total.
We normally only sail into Santander as it is a 24hr crossing and then go straight to Cabarceno Aire near the zoo in the village, only a 20 minute drive from the port, nice little village with a couple of decent restaurants and a couple of bars also a zoo, Free parking out of high season, fresh water available and grey/black water dump.
We always have a cabin so cannot give any info on the suitability of the lounge seating but I would imagine they would suffice for such a short crossing, we have not yet used the "economie services" but from info received they are very basic and for a little extra one can have some comfort on the other ships available.
You will need a French adaptor for the sockets for charging you electrical equipment.