Cross channel ferry to Santander (near Galicia)
Although you cannot travel across the channel directly from the UK to Galicia, but you can get pretty close by taking the ferry from Portsmouth to Santander (spain).
Santander is in the region of Cantabria on Spain's northern coast. It lies between Galicia's neighbour Asturias and the Basque country. Many vacationers wanting to take in northern Spain choose this travel option because it allows them to take their vehicle and tour the northern provinces of Spain with Galicia and Asturias being two of the main areas that they explore. Hopefully the links below will give you some cross channel ferry companies to investigate.
New Ferry developments for 2010
There are a number of conflicting reports about the availability of ferry routes from the UK to Galicia in 2010. One report suggets that the Santander route may close later in the year, however (as of now - May 2010) there are two (rather than one) ferries sailing this route.