Dogma, I'm afraid I have to disagree. The primary disruption I hear on 121.5 is airline pilots making inadvertent transmissions - everything from calling air traffic to calling company to talking to the passengers. The second is the local beacon manufacturers doing testing. The third most common use is airliners getting on to the correct frequency - often in the form "XXX this is YYY please call ZZZ on abc.de". That to me is an acceptable use of the frequency. Practice Pans come a long way down the list, and actual emergencies, thank God, come last. The first example is inevitable - we all make mistakes - but the incidence of it is evidence of a lack of care in radio selection. The second example is, I believe, actually criminal.
I really do think that 121.5 in the UK is primarily a GA / aerial work service in practice. Many years ago I used it most effectively to keep me out of the London Zone after getting a bit off track going round Luton Zone. I dare say that saved some trouble for airliners in the same general area. Airliners are already talking to an agency who knows who they are / where they are / what they are doing. That sounds to me to be a much better agency to call with your problem than D&D, who for all their expertise, don't know anything about you until you start talking.
Isn't the real reason for listening on 121.5 the same as mine - so as to be able to help in the event of an emergency? In which case, a pause of a few marching paces before stepping in would be sensible - wait out for D&D first, someone like me second, and if the poor guy with the problem still hasn't had a response, the words "MAYDAY RELAY THIS IS..." might just make somebody's day.
And to put all that in context, I do SAR shifts where we have to listen continuously to 121.5 in the ops room as well as in the air.
Sven