In the past decade or so have the English middle classes from the home counties started to use cockney slang ?

Question

and phrases, in order to gain credibility?

to the extend there is now only one, all-embracing, Thames-Valley accent,

Answer 1

There is some truth in this, and the result is horrible. Cockney slang goes with the Cockney. It cannot be emulated. Same as the Geordies - their slang is peculiar to them. Just listen to these cold callers when they telephone you - they all seem to have this horrible nasal copy of the Cockney. The true Cockney sounds great, because he is HE, and his attitude and demeanour are at one.