I assumed the same as racer_f50. They know the route which will be taken in advance so 2 or 3 crews go on ahead, and Jezza has an in-car camera setup. They meet him at his natural break points where they shoot any film of him outside the car if necessary (I.e. making jokes about MPG at the petrol station), and thereafter have a convoy for a few miles before he zooms off at single-digit MPG speeds
As for whether it is TV magic or a genuine fair race, I am not concerned either way, it is great to watch and very innovative TV.