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.
It's not so much the difficulty; It's the cost! The Auto Triangulation system in the UK has existed for many years on 121.5 and 243.0. Another Military UHF frequency was set up some time later to accommodate Practice Pans etc.
The UK Distress And Diversion Cells ( Military ) need the practice also, to maintain their own proficiency. They aid many GA pilots unsure of their position often in marginal VMC or worsening conditions and prevent them becoming major emergencies.
If the Airlines need another frequency to be covered; then, the industry will have to pay for it to be added to the system and pay for it to be maintained.