A couple of mates & I did the 'Coast to Coast' by Greenlane across the Pyrenees in 2002 with a company called Trans Pyreneean 4x4.
We took the ferry from Portsmouth to Bilbao, but it wasn't cheap, but it did save a long journey through France.
One of the other vehicles on the trip had taken to Dover - Calais crossing & had driven through France to get to our rendevouz in Sara in the South West of France & they were exhausted by the time they arrived.
Don't forget that when you get to the Mediteranean, you'll have a very long drive back if you're driving through France to Calais, then the drive in Britain before getting home.
We took 2 days to cross from the Med to the Atlantic coast by tarmac on our way home, then sat back & relaxed on the ferry back to Portsmouth (well I spent most of the time being sea sick due to the crossing of the Bay of Biscay being rough :->).
HTH