By air, then drive or train - You can fly to Bordeaux or Toulouse (both about 1 hour) or to Bergerac, then drive (about 2hrs from Bordeaux, 2hrs from Toulouse and 1.15hr from Bergerac). It is easy to hire cars from the airports. You can also take a train from Toulouse to Agen.
By train, then drive - You can take a TGV (train grande vitesse) from Paris to Bordeaux and Agen (then drive from Agen). The train journey takes about 5hrs, then its half an hour's drive from Agen.
By car for the whole journey - The drive from Paris takes some 7 to 8 hrs. You are on autoroutes the whole way until about the last hr.
By air then drive or train - Fly from one of the London airports to Bordeaux, Bergerac (around 1.5hrs) or Toulouse (2hrs).
Drive, cross the Channel, then drive – Drive to the port of your choice cross the Channel. There are two main routes through France on autoroutes for most of the way until about the last hr.
Train to Agen then drive – Take the Eurostar from Waterloo to Agen via either Lille (where you change onto a TGV) or Paris (where you have to change railway stations). Then either pick up a hired car opposite the station in Agen. Total journey time from Waterloo station is about 8hrs.
Airlines: Air France, Air Littoral, British Airways, Easyjet, Flybe, RyanAir.