Aog, did you know that there is a Jewish accent ? It's not Ron Moody playing Fagin. Jewish women in London, from traditional Jewish areas such as Hendon, have a distinct timbre and accent. It may be slowly dying out, because I don't notice it as much in young women. And men don't have the equivalent,or, at least, I can't hear one, but it can be heard in middle-aged and older women. At first, years ago, I thought it was just the universal accent of the area, but it isn't. And the explanation must be, as with other modes of speech, that it is acquired by exposure and copying of what the child hears, in a fairly tight knit community.
When everybody in the area tends to speak a certain way, black or white,they'll all eventually sound the same. When they mix freely with, or are overwhelmed, by a mass of people from outside their area, the accent will gradually die