Just back from France, too, in the 400H. On the main roads, limited to 80 kph / 50mph, MPG is amazing, 38-39. On the equivalent of a by-pass, still 32-33 mpg at 110kph / 70 mph. At 130kph / 80mph, MPG falls off drastically, 26-27mpg.
French roads are miles better than UK, depressingly. Driving standards are much higher and so traffic flows better, no lane hogs, overtake or stay in the inside lane.