I’ll let the Harrier experts comment on most of that but regarding Blair Atholl: There is indeed an airstrip there, and there is (was anyway) a big blue circle around it on everyone’s maps meaning avoid. Any temporary operating surface would need to be similarly protected. If the whole thing was too super secret to put on a NOTAM there would have been a red (Provost Marshal Protected) circle around it without explanation. I can’t remember either but there may be records somewhere.