Maybe It's Because I'm a Londoner -- Bud Flanagan (I think)
(My first thought was The Beatles "Because the world is round..." but the song is just called "Because", which wouldn't be very helpful. The song with "Jack" that came to my mind was "My Name is Jack" by some 60s combo, I forget who.)
The Moving Finger writes; and, having writ. Moves on: nor all thy Piety nor Wit. Shall lure it back to cancal half a line, Nor all thy Tears wash out a Word of it.