Hi Niknak.. Trust you're well. I had heard of such arrangements, but would doubt if it happened at Biggin without TC knowing about it. In the dark of the night there ain't much to do and the sudden appearance of something hefty on the radar would stimulate great interest. If it then disappeared over Biggin, reappearing a few seconds later people would get very interested and the matter would be investigated. Of that I am certain. If the Biggin aerodrome owners approved such activity it would have to be "known" by TC.
We used to have F-111s doing fly-bys at Heathrow in the middle of the night.. until one dumped fuel and lit it up with the reheat one night. Everything hit the fan big time!!