I would agree that the RAF aircraft stay at 8,000ft or thereabouts, although I have seen them in a direct descent from the loitering area back into NHT a few times at much lower altitudes. Same for the civvy frames that take over from time to time over the years. I would say this is 99% of the time, they stay at a set altitude doing their thing. However, in deference to your knowledge DRUK, I have seen non-RAF, but doing the dance, descend to between 2000-500ft and fly around for short periods before before heading off and up again. Perhaps a handful or two over the last 20 yrs. These are not taking aerial photos of a site of interest, not doing surveys, not doing air quality runs. They always seem to have a different quality to them, as though they are searching. Ultimately I don't know of course.
Perhaps as a web-track enthusiast you would be so kind to see what the Islander was the other day and if it had indeed descended from above (which I didn't witness this time) ?