I agree, the childish, moronic messing around on 121.5 needs to be clamped down upon. I do wonder how one of the frat boys in the front approaches the subject with the other pilot; "Hey, do you know what will make our job better? Let's belch and play Family Guy clips on Guard!"
"Sure, that sounds like a great idea."
"Oh, was that call for us? I was too busy being a prat on Guard to listen to your instruction."
As for a separate frequency for learner pilots to do their practice Pans on, that is a good idea in my opinion. I have no idea what spare frequencies there are across the UK that could provide a universal frequency. But I imagine it wouldn't be too hard to set up.