BUT it is not the practice to scabble at the switches immediately (ask ATC people),
Walter, Incorrect; it's irrelevant to ask "ATC people". It's not actually their responsibility what a pilot does in the cockpit in an emergency.
Again, please do bear in mind that I did operate in the SH role for about fifteen years. As an operational conversion unit instructor, I did know what was taught.
If there is likely to be no possibility of r/t contact with ATC, due to low altitude terrain masking, or distance (quite normal for SH), setting 7700 may be the one thing that alerts someone, somewhere, watching a secondary radar display, that an aircraft is in distress, without a word being said. This would hopefully result in SAR being alerted if necessary. That is what the 7700 code was designed for! The co-pilot, or the crewman may
well have attempted to do this.
if you had been flying the leg over the sea from Carnlough to the vicinity of the Mull low level, VFR would you have cruised above 750ft or below 250ft? Or was someone pulling my leg over generally avoiding 500ft to leave it for the FJs?
If I was flying below low cloud and I thought that military jets were likely to be in the area, I would have made a risk assessment and considered flying at the lower altitude at the expense of radar cover if seen as less risky.
SH is, by necessity, primarily a low level, VFR operation. It was by no means unusual to transit at a level below which the military fast jets were allowed to fly. In fact, we were directed to make best training use of "routine" transits by operating at the minimums. For example, during Cold War exercises, we were usually ordered to fly not above 150 feet above ground level, with the jets not below 250 feet agl.