As far as I recall (from doing it) you're not allowed in lane 3 on the toll roads with a trailer. Having said that, I've towed a racecar trailer down through France before and not had an issue, both with the lane 3 and *ahem* speeds.
As an aside, I didn't know there was a towbar available for the M5 ?? I didn't buy one after my BMW salesman told me it couldn't be done, because of the rear exhaust silencer being across the back of the car.