Interesting article.
Tri-rotors certainly exist, and indeed we were trying to get one working last August in Scotland. Basically a royal PITA, and about the only guy I know that got one working well is Swedish. So one at 1500ft? hahaha.
Something 2m in length is not going to be a “drone”. It could feasibly be a model rocket. I have a store room full of such things, and 2000ft-4000ft is exactly where they might be. But not at that distance from Stansted.
I don’t have historical records of our Notam info, but that reported incident would have been on the day we would have been launching near Twycross Zoo, so actually not too far from BHX. And I like to think that “we” know most high power rocketeers in the UK.
Having flown literally hundreds of rockets to those sort of altitudes, often needing other people to help keep an eye on it, I don’t believe for one minute that anyone could fly a drone at that altitude. Definitely not line of sight, and most probably not FPV either. And most definitely, not any “toy” drone that would have been unwrapped on Xmas morning.
All kinds of strange wind effects at those altitudes too.
Don’t have any historical launch info for high altitude balloons either. But the one site we (well acquaintances of mine) would use, would typically put them at something insane like 80,000ft by the time the overfly stansted. (Got an ace go-pro pic of that somewhere).