It's not any different than LotR Orcs having Cockney accents.
I think it's a tad more complicated than that. It's not actually Cockney but more a South London accent.
London criminals and gangs have traditionally featured heavily in iconic British TV police series such as the Sweeney, and other TV shows starring 'lovable criminals' such as Only Fools and Horses and Minder. That "Saarf ov the river" accent, (e.g. "You'll nevva get me alaave copper") has subsequently influenced British culture.
What is happening in the UK is that the South London accent is being used by criminal gangs far from South London, presumably in the same way as we have white teenage gang members talking like they come from the Bronx or Jamaica. This accent therefore becomes the 'go-to' for depictions of English speaking criminal groups.
