It's a good idea to find a pronunciation expert. You need a teacher for whom phonetics is not abracadabra. I know only three persons who are capable of teaching pronunciation; they are Piers Messum, Roslyn Young, and Carrie Terry. Watch the below playlist to find out more about English pronunciation.
You can't find this information anywhere else except a few books on phonetics.
My accent is RP. My first accent as a child was Cockney, from which estuary English and the modern Essex accent derive. I really cannot imagine teaching a student Cockney! I sometimes torture my wife with it.
I am not a specialist speech coach/pronunciation expert but I would be happy to have a chat with you. However, I reserve the right to try to persuade you that learning any of these accents is not what you need!