We go Portsmouth to Bilbao and drive straight down to La Manga these days but we onkly go by car and can drive down in 8 1/2 hours. You can stop half way at camp sites around madrid if you are towing a caravan. The year we went to Portugal first, we were in a motorhome and went from Bilbao to Salamanca then into the top of Portugal and stayed at Evora and then down to Turiscampo near Luz Bay. When heading towards La Manga after Portugal we went via Tarifa, Malaga and then on to La Manga. Depends on what you want to see on the way down, you can go through France and into Spain but if you are looking at cost it is just as well to get the Ferry Portsmouth to Bilbao because it can work out pretty well the same if not more by the time you pay fuel, tolls and camp sites. Hope you have a good time it will be the best thing you have done getting away from the UK winters and the cost of living is so much cheaper over there.
Angela