At the risk of reopening a long-dead thread, I would like to express a slightly alternative opinion to the majority of posters.
I don't think the the use of 121.5 to allow "practice pans" in the UK is a hazard to safety nor do I think that it should cease. The CAA recommends the monitoring of 121.5 by en-route traffic to enable contact to be re-established quickly if an aircraft flies out of range of the ground station, misses a change of frequency or suffers a sleaping receiver etc. Now, the majority of people find they have to deselect 121.5 because they struggle to listen to 2 active frequencies at the same time - ergo they have not suffered a radio failure!
It might be argued that once it has been deselected crews might forget to reselect 121.5 when the "practice pan" has finished. This is indeed a valid argument, but I think that most UK controllers would try a relay before going to the hassle of calling the D&D cell to get them to transmit to the offending flight.
If a crew has deselected 121.5 then suffered a sleeping receiver on box 1 then it isn't their day, but they should still spot something is wrong because for the most part of the day the UK frequencies are very busy and I would not expect to go more than 5 minutes without hearing a transmission.
DFC does raise a good point about operations on 121.5 in the UK interfering with adjacent FIRs. Surely this could be party avoided if guidance was given on where not to try "practice pans" (ie. not on the FIR boundary at FL235).
I personally believe that the large number of (seemingly spurious) ELTs which seem to transmit incessantly over 121.5 in some of the quieter parts of Europe are actually more of a hazard than a few "practice pans" in the UK. But that is only my personal view.
G W-H