Being a military Air Traffic Controller, one of my duties is to monitor 121.5
From my observation Practice PANs and Position fixes constitute less then 5% of the use of guard. About 85% of the time it is airline pilots inadvertantly transmitting on 121.5.
Sometimes it is funny to hear the reactions of fellow airline pilots when somebody does the brief for the passengers on guard in stead of on the PA
!
I gues about 10% of the time transmissions on guard are for real emergencies and loss of communications.
I am sure D&D has the correct statistics on the use and abuse of 121.5.
As it happens I am also a PPL instructor near Luton and with each of my students ONCE in their PPL training I request a position fix on guard. I think D&D are doing a wonderfull job and unexperienced (future airline) pilots temporary uncertain of position shouldn't be affraid to use this unique facility the UK has. Better one call on guard then to stack up ten liners in the hold for an airspace infringement!