This concept is enshrined in English law through precedent.
According to Wikipedia, it was first used in 1903, in a libel case, by Richard Collins, Baron Collins, who was Master of the Rolls at the time. Baron Collins attributed it to Charles Bowen, a junior defence counsel in the famous Tichborne Claimant case of 1871. (Charles Bowen became a Lord Justice of Appeal in 1882, and was awarded a life peerage in 1893, but died in 1894.)
No one knows whether Bowen had had a specific route in mind, much less a vehicle, but the likelihood is that he didn't; it was just a turn of phrase (Clapham being a fairly nondescript part of London, and the omnibus being everyman's form of transport).
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