I did this trip down to southern spain last year on my own. It was fine and I had a great time. The one thing that was invaluable was my satnav and my ipod! Talking books are quite good too. Be careful when there are no other cars around because that's when you forget to drive on the right.
If you are doing the peages have some cash in Euros with you and a credit or debit card in case something goes wrong. The other thing i did was buy a spare wheel for my car. It didn't come with one, and having shredded a few tyres by having blow outs in the past, I felt a bit too vulnerable to drive without one.
I avoided the peages as it was more fun and I wasn't in a rush. They are a pain as well because sometimes you have to get out and walk around the car to put the money in. I think if you take the T route through the peages there is a paymachine on the correct side but its card only.
Don't overdo it and try to drive too far in one go, and don't preplan your stops rigidly. I found on a couple of days I was fine to drive for 8 hours, but from Dieppe down the west coast before Spain I only managed 6 before stopping because it was a lot busier.
Always fill up with fuel just before you stop for the night or first thing in the morning. The fuel stations in Spain can be quite a distance apart.
Be careful where you leave your car because it is obviously british by the number plate and a bit of a target by thieves. Make sure you leave nothing in sight on the back seats and remove your satnav stand every night.
Check your insurance because some insurances revert to 3rd party only on the continent. You need a reflective vest, at least two breathalysers and a warning triangle.
Make sure you have at least four or five litres of water in the vehicle because if you breakdown, its very hot. I went down in June and it was 40 degrees going through the middle of Spain.
I think it cost around 40 euros altogether from France through to Spain on the peages, because I came back that way. If you are going in the winter be careful because it gets very snowy very quickly over the Pyrenees.
Have a back up for your satnav in case there is a problem with it. I have a map system on my tablet, and also satnav on my phone. Make sure you have a car charger for your mobile.
I stopped off at fairly cheap hotels, mostly chains, but expect to pay around 40 to 60 E per night. If you want to do it cheaper you can stay in the Formula 1s. I've heard they are fine.
There are patisseries in nearly every town and village for croissant or lunch rolls and they nearly always also do coffee.
Its a great trip to do and I think you'll have a great time. Brilliant fun and a good way of seeing France and Spain. Hope you have as good a time as I did.