Tunbridge Wells is a real town, and this expression derives from the name. This Wikipedia article relates the story:
The term Disgusted of Tunbridge Wells is a proverbial sign-off-name in the United Kingdom for a letter to a newspaper complaining (often excessively) about a subject that the writer feels is unacceptable.
The term apparently dates back to the 1950s. Historian and former newspaper editor Frank Chapman attributes it to the staff of the former Tunbridge Wells Advertiser. The paper's editor, alarmed at a lack of letters from readers, insisted his staff pen a few to fill space. One signed his simply "Disgusted, Tunbridge Wells".
The term was given a wide currency in the early 1950s by the BBC radio comedy series Take it from Here in which Disgusted, played by Wallas Eaton, would make a ludicrous protest to give the cue for a sketch by Jimmy Edwards and Dick Bentley.
In 1978, Radio 4 called its new listener feedback programme, now called Feedback, Disgusted, Tunbridge Wells.
So concerned of Tunbridge Wells refers to a concerned resident (figuratively) of Tunbridge Wells, as the name has come to connote critical feedback. The disgusted form outnumbers the concerned form on Google by about 5 to 1.
As to the last question, I would say that it is quite unlikely that the concerned form is anything but an ameliorated conflation of the disgusted form; it carries the same meaning and follows the same etymology.