going to bore people with details, ferries and travel to matienzo.
economy versus practical.
if driving the best way to my mind is Brittany ferries Portsmouth/Plymouth to bilbao/Santander . it is also the most expensive option.
the next economy wise is to fly/hire car.
now for economy versus mileage versus time. when you do the sums of distance saved from going Portsmouth caen/st malo, versus cost and time on the ferry, to me does not make any sense to go this route, mileage saved is minimal, time lost due to time on the water is far greater than driving Calais down.
econmy is Calais driving, ferries/train are very competetive far competetive than Portsmouth or even Dieppe route. going caen saves less than 200 km, and even on the quick boat is 4 hours. from north to south distance to Portsmouth or dover is very similar. you can get an early evening ferry and with 2 drivers be in matienzo mid morning. so for me I can not see any advantage of using any of the channel ports, you don't save enough mileage to make it worth the time lost on the sea and the extra expense of the crossing
from sussex I used 1 1/4 tanks of fuel to reach matienzo. going from dover. approx. 1000miles, and only the little bit before dover and the bit from coast to matienzo is not on motorways, you could also stop over some very cheap rooms can be had if you look around and book in advance