I'm reading all your trip to France threads with envy.
How far south do you have to drive to be assured of sun/warmth in July? I'm asking because I'm in north west England and so any trip involves an additional day each way to/from Dover 
I've looked at ferry prices from hull/Newcastle but it's £500... but if it's possible to get warmth not far from Amsterdam/zeebrugge then I'd definitely consider it.