Click on the picture to see a larger version.
The hackney coach driver in the joke was apparently attempting to cheat the magistrate since the legal fare in the 1700s was a shilling (12 pence) a mile and the distance from Westminster to Charing Cross is less than half a mile.
Source:
A Joe Miller joke illustrating the usage of "set down".
Joe Miller's Jests or, The Wits Vade-mecum, with a new introduction by Robert Hutchinson. New York, Dover Publications, 1963. Facsimile republication of the 1739 edition.
Bloomsday for Cab Drivers / 12
Time and Setdown / 2
As ingenious and plausible as Michael Higgins's explanation of "time and setdown" is, there is a simpler one that is perhaps closer to the experience of actual cab drivers.
Back in the 1700's "set down" was a hackney coach term meaning to unload passengers at the completion of a trip. "Set down" in this sense appears in the 1739 edition of Joe Miller's Jests as well as in London cab legislation.
Gradually "set down" came to mean the trip itself: "for sixpence one may have a set down, as it is called, of a mile and a half, and a tumble down into the bargain" ("Noddy" in Lexicon Balatronicum, 1811. The noddy, originally a farm cart with seating for passengers, was the forerunner of the Irish jaunting car.)
From this we might interpret Leopold Bloom's observation to mean simply that the drifting cabbies are not only subject to all weathers and all places; they are also on call at any time for any kind of trip (setdown).
All this simply illustrates how easy it is to get into a debate about the true meaning of Joyce's text especially when the "drifting cabbies" passage is, by Joyce's standards, pretty clear.