I'll reply to the original thread, then read the rest
I did this, or similar, 2 years ago. It cost me around £700 in fuel in a fully loaded M coupe (including around 200 euros worth of tolls we chose to use), but that included a few spirited days in the pyrenees. I averaged around 27mpg through the 2800 miles we did (London -> Valencia, and back, via some random destinations + mountain roads).
Get yourself a pay tag here: https://www.saneftolling.co.uk/ - (just hold it to the windscreen when passing, dont stick the bugger down - still cant get mine off..!) give it some time to be delivered (whatever the webby says). This will stop you having to stop and pay at each toll booth with change you dont have etc - also skips huge queues (most of the french seem to not bother with these so you end up with your own premium lane! hilarious the amount of hours we saved). Worth mentioning the tolls roads can be SUPER fast, on teh way back we did Carcassonne -> London in 1 day, toll roads were invaluable for this, but if youre happy crusing around you dont have to use all of them.
French police used to demand you had a breathalizer cartridge/tube set, i think this was dropped as a requirement but we still carried it. You need warning triangle + high viz vests.
We did the Eurotunnel, no problems - they actually put me on the truck/supercar carriage for clearance which gave some wider carriage space.
Other observations: the french are 10x better at lane discipline than us. The french love Z4M's, all their cars are .. french and slow. The Spanish have normal cars, and dont care.
If you're looking to do some stops on the way (rather than blast most of the way there) i'm sure we can all recommend some places, have been holidaying/driving around central/southern france since i was a baby. IF you want a funky camping stop overnight before crossing the border towards Barca, checkout Belrepayre Airstream Camping - funky place, cool couple that run it.
Oh - and remember 2k+ miles is a lot - check oil (maybe change it if you plan some spirited drives), and tyres.. i replaced my rears before going with some fresh PSS as i knew sub 3mm wasnt going to last my idea of mountain road cornering...
