Walter, the losses on Blenheims at Church Fenton were reflected across the country at other OTUs, Bicester included. Could the common carrot have been a convenient scape-goat for the handling limitations of this aircraft? It had been at the very edge of aeronautical technology in its time, but that time was too soon for WWII of course, as was quickly apparent in front line use.
Whether its use thereafter for training purposes was for the best is debatable, but there was a war on and it was by then the only real use to put it to, and anyway it could be argued that if you could learn to handle a demanding training aircraft the easier you would find the operational one. In the meantime you were probably put off carrots for life!
Thank for the thoughts on the camera obscura. I guess that the signal, in whatever form, to the ground team was for the training staff. They simply had to wait for the trainee to initiate the moment of simulated bomb release to do so.