In reply to Justthisonce, active sites GA/parachuting/gliding are marked as such on aviation maps, generally the military avoid them because A) they are aware and logistically it makes no sense for introduce potential conflict and B) A FJ doing 300 knots in non-advisory, Vfr mode is unlikely to visually acquire a thermalling glider as it transitions from head-on to side in the turn in time to take avoiding action.
I've been on both sides of the coin having unwittingly flown through Sibson whilst it was active, my bad, I assumed that the low cloudbase and wind would have precluded parachuting. I got a call from a third party, rang Sibson and apologised, no harm, no foul but my bad.
I've also just come off the winch at 800ft at the Mynd and found myself in albeit brief formation with 4 or 5 hawks, again call to Valley, error acknowledged and very apologetic. Suspect this was a Navex that went slightly astray, all I saw was Hawks suddenly going vertical and disappearing into cloud. Winch operator was more concerned than I as he saw them briefly and thought they were below me, i.e. potential tow cable conflict.
I'm not disagreeing that UK airspace is busy but F-15's have enough autonomous guidance that this should have alerted the pilots (assuming their software is correctly updated). As a GA pilot, I wouldn't dream of pottering down the mach loop because of the potential conflict.
I've got to disagree with the board's decision, yes they couldn't have seen them and be expected to take avoiding action, however prevention is better than cure and they should not have been in that airspace period. IMO, there is enough non-restricted airspace that the FJ's could have avoided this without compromising their mission.
We've had enough mid-air's where it was just wrong place, wrong time without multiplying the factors by flying through known active sites.