'As I explained to you all before' BBC English is that NEW STUFF that came along when Aunty had to speak in a style that would be understood by a wide audience. It is sometimes called "Educated Southern English".

Cockney is (and was, long before the BBC)spoken by the people of London. That is, in or around the City.

Essex has a dialect all of it's own. Remember that old invoice for taking care of a horse "A fetchinonim, a feedinonim, and abringinonimomeagain."

I don't know about S. Middlesex but most people in the Greater London area now speak some version of Cockney and can be recognized as Londoners.

Each of the Home Counties used to have it's own accent, which can still be heard in some villages.