The mil introduced a 'Practice Emergency Training Frequency' (PETF) to reduce the loading on UHF 243.0. The main reasons were to prevent Pilots 'tuning out' from 243, but also to ensure the 243.0 frequency was available for a genuine emergency when it was really required.
Frequencies are scarce in the UK, but if D+D training is considered to hold such a high Flight Safety benefit then a VHF one could and should be made available. Both the reasons that caused the Mil to create a PETF are equally valid for 121.5