I use Fleet Watch on these registrations:
G-SAR* = Search and Rescue
G-CG* = Coast Guard
G-FR* = FR Aviation (military support)
G-SAS* = Scottish Air Ambulance
Z* = RAF (but also some others)
Or, you can sort the list by altitude, since most of these fly at low altitudes. Or, filter on Lattitude = 0, since most of these are not ADS/B equipped.