I know you said Calais but...
I would say to look carefully at changing route, come down the M1 to Northampton, take the A43 past silverstone to the M40, along one junction and pick up the A34 to Winchester, M3 then M27 to Portsmouth. Northampton to Portsmouth is 2 hours that route. Ferry to Le Havre (excellent Patissiere next to the round white concrete cinema in the town, turn left off ferry, almond croissant to die for, great breakfast).
French route Le Havre, Pont L'Eveque, Liseux, A28 to Le Mans, round Le Mans, Tours, Loches, Buzancais, Chateaurault, motorway with no tolls all the way to Souillac, then you're nearly there. Its a good route, I used it this year and last.
If you want to stop off half way, just a few miles south of Blois is Chitenay, Auberge Du Centre is very very lovely (
http://www.auberge-du-centre.com/fr/sommaire_fr.html ) with excellent restaurant and eating in the garden with jazz, good singer, pianist, marvellous food, try Chevernet local red. Splendid!
Chartres also worth a lunch stop on the way, stained glass in the cathedral is worth seeing, even for a confirmed atheist like me (did you see what I did there?)
If you want to stop near Le Havre then Honfleur is lovely. Hotel Dauphin very nice, parking not great though.
http://www.hoteldudauphin.com/ Moules and frites on the quay.
From Doome, canoeing on the river is good fun, Gauffre de Padirac cave is worth seeing as is Rocamadour, make the climb to the very top.
Nip down to Cahors, nice market to trap tourists but worth looking at Pont Valentre, interesting history, see if you can spot the carved stone devil climbing up. Perhaps not too interesting for kids.
It was like that when I got here.
so thought I'd try & use your collective wisdom 
