I use the ferry from Plymouth to Santander regularly. The bus from Santiago de Compostela usually leaves at 9am and takes around 10 hours to get to Santander which is in good time for the 9.15pm sailing to Plymouth. The first morning bus from Finisterre is around 8am and takes about 2 hours, so you would need to get the bus from Finisterre the day before and spend your last night in Santiago before heading back to Santander. The bus from Finisterre to Santiago is around 10 euros, and the bus from Santiago to Santander is around 50 euros. For both buses you need to get your tickets in advance. At Finisterre you get it in the kiosk opposite the bus stop. In Santiago you get it at the Alsa Bus kiosk in the bus station.