Giles,
When I am at FL350 just in the Paris FIR, I may be receiving an emergency call from a low level GA flight out of range of ground stations when your practice pan blocks out their call!!!
Using 121.50 for practice is just the same as using 999 for practice. I would hate to be the one that blocks the emergency frequency causing some pilot unnecessary dificulty.
I feel that this all comes down to money!
NATS piggyback their responsibilities for monitoring 121.50 onto the military system. To reduce the requirement for simulator training with the D+D staff, they get people to use the real frequency for training purposes. What a shame that the 243 coverage is far better than 121.50 despite both being provided by the same people!!!
Regards,
DFC