I don't think anyone is saying that an investigation is unwarranted, just that it is not compulsory. This event does not fall under the remit of ICAO Annex 13, so the AAIB does not have to get involved. The aircraft is not registered and does not have a CofA or permit so the CAA does not have full jurisdiction. It would be interesting to look into this, but the 655MaPS has already provided a story from the flightdeck explaining what happened, so it's not as if there's a whole lot left to uncover. As for all the questions wondering why they didn't do it differently, my view is that as they have been doing this for a long time, the operators, together with the airfield authority, are those who are best placed to figure that out.