I recently <cough>obtained copies of the BBC's production of Dorothy Sayers' The Man Born to be King
in MP3 format. They're pretty good.
Also, if you're interested in radio dramas that are still in production, http://www.decoderringtheatre.com
has some good stuff. I like their Deck Gibson stuff best, but it was a short run just for the summer one year. Their best ongoing series (IMHO) is Black Jack Justice, but the Red Panda is pretty good to.