Would the "Cavendish drawl" be considered a dialect?
What about a minimal dialect:
A dialect spoken by one individual is called an idiolect. Everyone has small differences between the way they talk and the way even their family and best friends talk, creating a minimal dialect.
Source: Sociolinguistics lecture slides
Slang, jargon, and cant are terms used to refer to words used by small groups or sections of population. The Devonshire drawl was more like Valleygirl speak than a dialect. They were a group with their own jargon used to keep outsiders out.