I went on a tour at London Centre D&D at West Drayton a couple of weeks back, and they have quite comprehensive monitoring and triangulation facilities for 121.5 across the whole country. They also have landlines to all the major ATC units so if they need to warn off GA traffic from a specific area, they can do so with ease.
I've done several 'practice PANs' over the years which show how effective they are, although I had to laugh on one occasion, where amidst the sound of tumbling coffee cups I was directed to leavesden (in a fixed wing) about five years after it had shut !
I'm not sure how much added benefit GA monitoring of the frequency would provide.