This expression became common in the Middle Ages during a time where majority of the merchant class was Jewish, making the Jews among the most wealthy of the time (Dictionary of Phrase and Fable: R, 1894).

Above is a postcard that illustrates what the Union-Passage looked like at the beginning of the 19th century (Sanborn, Vic, Jane Austen's World, 2010).

Isabella is referencing The Horrid Mysteries, A Story from the German of the Marquis of Grosse, a translation by Peter Will of Der Genius. The original is a German Gothic novel by Carl Grosse published in 1796 by Minerva Press (wiki, Horrid Mysteries).

The story's plot synopsis is as follows: "The hero of the tale, the Marquis of Grosse, finds himself embroiled in a secret revolutionary society which advocates murder and mayhem in pursuit of an early form of communism. He creates a rival society to combat them and finds himself hopelessly trapped between the two antagonistic forces. The book has been both praised and lambasted for its lurid portrayal of sex, violence and barbarism" (wiki, Horrid Mysteries).

"The large parish of Tetbury, comprising a prosperous market town and the small hamlets of Upton, Charlton, Doughton, and Elmestree, lay on the southeast boundary of Gloucestershire, 9 miles south-west of Cirencester" (BHO).

The actual distance between Bath and Tetbury is 20 miles (Google Maps).

This is the third novel by Frances "Fanny" Burney, originally published in 1796 (LibriVox).

"This is the story of Camilla, her beloved but selfish brother Lionel, her sisters Eugenia and Lavinia, and their extremely beautiful but thoughtless cousin Indiana on the months proceeding their marriages. Camilla is deeply in love with Edgar and he loves her back. However, on the advice of a friend, decides to make sure that she is free of fault. She has the luck to find herself in lot of uncomplimentary and comic situations which doesn't make Edgar's wish easy. Meanwhile, Camilla, on the advice of her father, is trying to make sure that Edgar really loves her before marrying him" (LibriVox).