DR Jrand75, I believe the play you saw was Alan Ayckbourn's A Chorus of Disapproval. I've never read it, but I saw the movie adaptation with Jeremy Irons and Anthony Hopkins. The play within the play is not Brecht's Threepenny Opera but its 1728 inspration The Beggar's Opera.
And while I'm on that, let me recommend Peter Brook's wonderful film version with Sir Laurence Olivier, Dorothy Tutin, and Stanley Holloway, additional dialogue by Christopher Fry and music arranged by Arthur Bliss. From info in Benjamin Britten's collected letters, I gather the original intention was to use his 1948 score for The English Opera Group, but I suspect his wonderful, but very modern arrangements, scared off the production. The BBC video from 1963 is also interesting and well done.