Just seen this thread.
Trains: Lille to Valence works, but for less hassle all round I'd just go Eurostar London-Paris, then it's two stops on the RER from Gard du Nord to Gare de Lyon (direct), then train TGV from Gare de Lyon to Valence. An hour between your train arriving GdN and the one leaving GdL is plenty. And I think trains from Lille only go to Valence TGV (which is 10km outside Valence). There are two stations, Valence TGV (2hrs10mins from Paris) and Valence Ville (2hrs 35mins from Paris). Most TGVs these days go to Valence TGV, but from Gare de Lyon there are a few each day which go to Valence Ville.