Mollie, for some of the really popular sites people make advance bookings for the following year before they go home and many take new bookings from October or so (basically when the current season ends).
No idea where to go other than I think it might be nice to have a go at camping in the sun . We were lucky with the weekends we did this summer but we only booked one or two days before we went so we were able to know what the weather would be like. I'm wary of booking something a long way in advance without the hope of good weather. I figure we might be luckier in France than in the UK. Also it will be good for ds to practice his French.