just done this journey for the first time at the end of august and returning 2nd week of october - (essex to villamartin).
1 driver, me, 1 passenger, 1 dog.
left home at around 4am to get le shuttle from folkestone. caught the 6.20am one. then with only 3 stops for loo, coffee and dog break arrived at our overnight stay in bayonne southern france at around 7pm and 740 miles later. bit too long that was and next time will add another night stay i think.
left bayonne around 8.45 the next morning and arrived in villamartin around 6pm, no issues and easy drives. stuck entirely to motorways and the tolls in total for the return journey was around €175. all driving (apart from 1st leg to folkestone was done in daylight, which really helps).
next time i am going to drive from home to nantes. stay overnight, then drive to pamplona, stay overnight and then down to villamartin. so around 400 odd miles a day. if my wife could help with the drive then 1 overnight is not an issue but as the only driver i found 700+ miles a bit too far in a day. the second day down to our house was around 580 miles, so much more doable.