Have been involved with a few of these underslung car jobs. Saab into a dealership in Aberdeen, Peugeot into a remote photo location on Skye, Hyundai onto the white cliffs of Dover. One was the launch of the Rover 800 (anyone remember these?). Had to fly it from Glasgow airport up the Clyde to the Rover dealership. As we transitioned away from Glasgow airport, I heard ATC instructing a 737 to "line up and wait after the departing Rover".
Don't know if it is true or not but heard a story that Bristows were asked to undersling a new Citroen from Calais over to the UK in the '60s using a Whirlwind. The story goes they got half way across the English Channel and realised they didn't have enough fuel to get to the coast. To far out to return to France so they jettisoned it !