It was by mere chance that I found these two quotations, the first of which is take from the novel Coming up for Air written in 1939:
The second one is to be found in an essay with the title "The English People", written in 1944, but not published until 1947. Among other things Orwell writes about steps towards greater democracy in England. Here is his description of the third step:
Doesn't this sound - at least vaguely - like an anticipation of the concept of Estuary English?
There might even be a link to what Sidney Wood writes in his blog about a personal experience he had at school:
Of course, he [= Rosewarne] couldn’t know that our English language master had already told us at school in 1950 that our dialect was Estuary English. It seems I’d been speaking Estuary English for 50 years before Rosewarne coined the expression. Perhaps it was our English master who coined it, or perhaps he’d read it somewhere. If so, it would be exciting now to know where.