The warmer climate only comes south of the Pyrenees, so France is not much fun right now. You can figure it out looking at a map, but basically France has 2 main west to east corridors, one from Paris to dijon, and one in the south from Bordeaux over toulouse to perpignan.
You have pretty much 2 options then, if you go first past paris (St Malo-rennes-paris-dijon), its not so direct, but you will have the chance to pass a lot of cities, which I find a nice choice when hitching in the winter. In the eastern corridor is dijon, lyon, montpelier, perpignan... Nice big distances to hitch, with cities to break in between if you need food or rest. I think its also a lot easier to catch rides over here, you will find a lot of international drivers heading for spain.
On the other hand, the west is a lot more beautiful, and less populated. If you want nice nature, great coastline, and tons of quiet, its better in the west (I wouldn't recommend it in the winter though). There is quite some big wave surfers on the west coast in the winter, which might turn into a funny ride here and there. From bordeaux cutting east to perpignan, near to the spanish border you will cross if going to BCN.
I reccomend finding a ride from Perpignan to BCN, even if it means waiting a bit longer, as once you get into spain the hitching gets immediately more difficult. Truckers generally dont take more than one rider, and spanish people are often scared to pick up hitch hikers.
Take care around the border, there is a lot of sketchy shit happening, and getting stuck in Jonquera (the border town) is no fun. Likely to get robbed, lots of violence, and cops everywhere. You can find a ride here, but better to be quick away if you get stuck, otherwise you might deal with the police.
When you get to BCN, check the infousurpa, the schedule of events at the local squats
(usurpa.squat.net/)
I have made this trip countless times, as I have lived in spain and portugal for the last 3 years, but always coming north to visit friends. The maps of Frace will show a lot of different crossings, but France is fucking full of mountains, and you dont want to go that way this time of year. Just saying that your idea of going south east might be better to split into east, then south, just for comfort. South east from St Malo is directly into the mountains.
Drop me a line if you have more questions, or need site specific info for any place, always happy to lend hand