It depends what week you consider half term, but expect France to be heaving 19/2/10 and 5/3/10. Much as I love skiing in France I would give it a miss the last 2 weeks of Feb and the beginning of March. Instead concentrate on the other countries (I know little of how busy they get).
As Boabski says, the first week in January not including New Year is the quietest week of the season. It then gets progressively busier until early March when it drops off to May. New year week is also extremely busy
I am not sure what you mean by a reasonable driving distance, but assume you mean less than 10 hours from Calais (like La Plagne). I have a friend who pointed out to me that if you live in the south of England than it is almost as fast to drive to St. Anton as to fly so do not restrict yourself entirely to the western Alps.
Anyway my suggestions to examine include: Champuluc, Zermat and Verbier. Though even in quiet weeks I have experienced pretty long lift queues in each of these resorts. If you really fancy sticking to France then some of the extensive, but little known resorts such as Vars or Les Sept Laux may be worth looking into.
Basically expect anywhere in France to be busy from Feb 19th for the next 3 weeks
John