In reply to phizz4:
> Luxembourg (cheap motorway fuel)
Check the fuel prices before you go out of your way. Last time I went, there was only 7 cents difference between diesel in Luxembourg vs Carrefour in France. Even if you manage to time it well enough to fill the tank, it's only a couple of quid.
Don't know where you are in the SW, but I would seriously look at Plymouth-Roscoff overnight if you can get a crossing in budget. Saves you making what would be a crappy journey to Dover, and you would get to avoid the busy Autoroute du Soleil in France. (I'm in the NE, and I'm a total convert to a good sleep overnight on Hull-Zeebrugge followed by driving all day.)
viamichelin.com will give you toll costs. As a guide, I think I spent about €80 in tolls each way last year from near Lille to Cham. A lot of people say you won't save much by staying off the motorway, because you'll end up using more fuel on congested national roads.