I will never understand why ANYONE would misuse the 121,5 for anything except a legitimate emergency/PAN!
Below is an extract from CAP 413:
1.7 Radio Procedures – Practice Emergencies
1.7.1 Pilots may simulate emergency incidents (BUT NOT THE STATE OF DISTRESS) on
121.5 MHz to enable them to gain experience of the ATC service provided.
Using 121.5 in the UK from the above reasons is legitimate, documented and approved. It's a good way for PPL's etc to practive what they would do in a real emergency, so that they have the confidence if they even need to do it for real.
It's good for controllers, so that they can keep current and practice the relevent procedures.