Loosely speaking, converting the camera coordinates to real-world coordinates allows you to locate the UAV relative to the camera position. You know the LEDs are 400mm apart along a shared edge, so you can scale the pixel distance in your imaging to that distance.

Depending on how much detail/nuance you desire to introduce, you may wish to take the orientation of the UAV into account if it is not oriented perpendicularly to the ray between the UAV and camera.

To locate the distance away from the camera, you will need to know the angle subtended between the rays of neighbouring LEDs. If you know the FOV information of your camera, you should be able to calculate this angle and use trigonometry to calculate the distance between the camera and UAV.