If the Tapley meter is not attached to the heavy frame that holds it in place on the floor of the car, and is in fact hand held, it is quite a small object. Possibly easy to miss if you were looking for something larger. The operating instructions for the meter require the vehicle to be driven at a steady speed greater than 30kph and then the brake/s applied smoothly and firmly (without "jabbing").
I doubt that this procedure could be safely carried out on the service agents premises, hence the need to go out on the road. Not defending anyone, just saying brakes are a kind of important safety feature on a vehicle. So the service agent would be reasonably keen to make sure that they were working properly before putting a WOF on the car.
Cheers...