By no stretch of the imagination does Cameron's use of the word suggest a neutral or positive definition of the word.
The fact that Mr Green has lived 90 years without ever hearing this word, sort of disqualifies him as a judge in this case. Since the context of this use of the word is negative, here is what the Macmillan dictionary has to say: PLEB: British informal. An insulting word for an ordinary person show[ing] that the speaker thinks ordinary people are not important or intelligent. COMMON: insulting way of describing someone from a low social class. Here is a direct quote from the police log: Mr MITCHELL said: Best you learn your f------ place...you don't run this f------ government...You're f------ plebs. It sounds to me that it is Mr Mitchell who has not learned his place in a democratic society.