I suppose the short answer to 'berk/bark' is that Cockneys seldom use RP. It's interesting to note in that respect that what constitutes a London lower- class accent seems to have changed radically since Dickens' time, say the 1830s/40s. Sam Weller particularly, and low- life London characters as in Oliver Twist etc., show no sign whatsoever of rhyming slang, in fact the main characteristic noted is a V/W inversion ("werry good, Sam Veller"). One assumes Dickens knew what he was talking about, he lived among them.