I’ve always assumed that “but I digress” is a quote or catchphrase from somewhere. It has a certain mock-formal, humorous ring to it. Sounds like it could be a Dave Barry-ism, say. This particular form certainly seems a lot more common than other ways of stating the same thought, such as “but I’m digressing”.
It is attributed to US comic author Max Schulman (Dobie Gillis, Rally Round The Flag Boys, The Tender Trap) in cigarette advertisements of the 1950s.
It is used by Tom Lehrer in the song “In Old Mexico”, which was recorded in 1959.
Thanks both. It defeated my Googling. It seems that Max Schulman’s original wording may have been “however, I digress”, but even knowing that I don’t think it would have been easy to trace.