I've been interested in many of the posts on this thread and on similarly related ones on pprune. If I may add a bit more to my previous posts on the subject I hope I'll be able to clear up a few misconceptions and, perhaps, provide some information:
D&D in the London FIR, primarily, have access to three frequencies. They are 121.5, 243.0 and 245.1. All three have DF facilities dotted around the country. It may be possible (since the move to Swanwick) to select other frequencies on the kit in D&D but that will not be tied in to any DF. 245.1 is PETF - Practice Emergency Training Frequency - for the military to do their practice pans and training fixes on; thus not interfering with 243.0. There is no VHF equivalent PETF. If the guard police feel so strongly about this lack of a VHF PETF perhaps their airlines should lobby nats to provide D&D with one (and maybe help to fund it). All of the controllers in D&D are RAF. Their orders include the handling of practice emergencies on 121.5. The only times when they will not allow training calls on 121.5 is during actual aircraft emergencies (sensible enough) and whilst SAROPs are in progress during which the SAR crews require r/t silence on 121.5 so they can home in on ELTs/PLBs. Read that bit again, guard police. "..... the SAR crews require r/t silence so they can home in on ELTs/PLBs".
I hope this provides a little more of an insight to those not familiar with the workings of D&D.
I will be very interested, as I'm sure many others will be, in the outcome of a survey that was conducted at London Centre throughout May. It was commanded, by those much higher up the "foodchain" than the D&D controllers, to illicit exactly how much time was taken up on 121.5 by inadvertant transmissions, abuse, actual emergencies, practice emergencies etc. Of course, because of the radio equipment, not all transmissions were made whilst the aircraft were in the London FIR/UIR, but the survey will have logged all aircraft calls on 121.5 that were heard at Swanwick. I admit that I don't know what the purpose of the survey is - still, I'll bet that the time taken up by misuse/abuse and inadvertant transmissions far outweighs that of the practice pans/training fixes.