I remember the 'old' Experimental Flying Control Assistants at Farnborough all got an Imperial Service Medal when they retired as a matter of course.
The ISM is more of a "long gong" for Civil Servants than an award.
Unlike the LSGCM, it is still for 25 years "undetected crime", but - IIRC - is only awarded on retirement. And it's only for D (maybe E?) grades and below I think, so - once again comparable to LSGC - not available for "officer equivalents".