my parents live in perpignan, and their friends live in perello, so have done this journey a few times.
favoured route is a26/a1 calais-bethune-cdg then around the east side of paris to the a10, then a10/a71 paris-orleans-bourges-clermont, then a75 across the massif central (including viaduc de millau - awesome!) to a9, then a9 beziers-narbonne-frontier, then along the motorway in spain via girona to barcelona.
if on a day or time when paris area could be expected to be busy, then a16/a28 Calais-Abbeville-Rouen, then a158/n158 Rouen-Evreux-Chartres and re-join the a10 at orleans, then as above. This diversion adds an hour to the journey, but is quicker if paris is busy.
where to stop depends on how far you've got to drive before you reach folkestone. But Calais to Orleans is about 5 hours, Orleans to Clermont about 3 hours, and Clermont to Perpignan between 4-5 hours depending how busy the a9 is. Barcelona is about 3-4 hours from the border.