Straight down the A3
.. to Portsmouth, then Brittany Ferries to St Malo, check out Mont St Michel for the touristy bits, then head down the Atlantic coast to the Pyrenees, over them into Spain and back up from Santander to Blighty. That should give you a decent tour, you'll see the countryside and architecture change as you move down the coast (it really changes at Nantes), you get the more unspoilt (and cheaper) Pyrenees over the Alps, a taste of Spain, cheap duty frees in Andorra and the boat takes the strain of coming back up.
Only downside - Brittany Ferries - they're not the cheapest ferry operator across the Channel, but book the right times and you can get bargains.
(Hmm used to go to school in Guildford)
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