In my airline training (2 places), both said all distress communications should be initiated on the frequency already in use, and only then should 121.5 be used if no joy obtained. So in the UK, unless you believe there is a chance of getting shot down by the miliatary with no visual warning first (ie unlikley), then 121.5 should be virtually redundant for commercial pilots.
Meanwhile, you have UK guys on their first cross country VFR NAV exercise potentially getting lost, unaware of any available radar coverage, and risking IMC conditions or fuel starvation. SURELY these people should actually get priority, when there is no-one else with a radar for them to call?
Over the Middle East, or Africa, or elsewhere I can understand its a different story. But not in the UK.....
IMHO