Hesse's Aircraft Was a Hostile Intruder

If Hesse's aircraft had been expected, RAF fighters would have been sent up to escort it safely to its destination. Three Spitfires and a Defiant were ordered to attack and not escort the approaching enemy aircraft. They'd already dispatched four aircraft to shoot the Hesse plane down. That means four friendly aircraft were giving chase to the plane - you tell your anti-aircraft batteries not to shoot at a chase like that. If your ground-based batteries start shooting, you're likely to kill your own pilots with friendly fire. So, this argument that the British were expecting Hesse is not a good one.