It would appear to be too difficult for the CAA to either fund another frequency for training fixes in the UK or, indeed, cannot find one that is not in use already. D&D are already monitoring two frequencies 243.0 mhz (Mil) and 121.5 mhz (Civ).
Mind you, even if Guard was left for Actual Emergencies only, They would still be distracting to those not involved.
Perhaps, the only other solution is to have a 3rd person in CAT aircraft to monitor 121.5. I guess in the old days; a Flight Engineer or Third pilot could have done so leaving the other pilots to concentrate on box one.
Can't see the bean counters liking that one though!
(Edited to say I was unaware of a mil training fix freq. I left the RAF in 1991 - so obviously out of date now!)