Brummies don't exist, or at least are not allowed on TV.
Corrie, Enders and Emmerdale (none of which I watch) all regularly feature characters from London, Manchester, Yorkshire, Liverpool, Newcastle, Scotland basically every accent is covered except for the one which must be the second most represented accent in the country.
Peaky Blinders?
This was a freak event. It was created and written by a Brummie. he must have got the show commissioned set in Manchester then switched it pre-production.