I have flown UK HEMs/Police for 8 years. My spare VHF box is alway set to 121.5, if I'm not using it to call ahead.
I'm biased though; many years ago when I was a new boy, flying along solo in a Gazelle, I heard a Mayday call on 121.5. I waited in anticipation to hear the D+D system wake into action!
Total silence.
I was the only person to hear it!
(Hell! How does that "Mayday Relay" go??!)
Found a light fixed wing with engine failure, landed safely.
(I got a bollocking for saying the word "Mayday" on the Army Tac net!)