In Great Britain (and to a large degree in Canada, Ireland, Australia, New Zealand and South Africa) “pleb” is used as a derogatory term for someone thought of as unsophisticated or uncultured.
Tories are considered an “upper class” party representing the “posh” (another annoying term). So this was seen as “Their Lordships spitting on the servants” kind of episode.
Think of it as another variation of the alleged “47%” scandal
I’m guessing the term “pleb” comes from “plebian”. Roman society was divided between patricians & plebians. The insinuation of “commoner” is obvious.
“Posh” on the other hand, is an acronym meaning “Port Outward, Starboard Home”. This referred to first class accomodations on British passenger liners where those who could afford it booked cabins with southern exposures both ways.
Interesting to know that class distinctions are alive & well in the mostly socialistic U.K.