Oh yes! Although, it's Luther's boss who speaks with such an overdone Cockney accent. It sounds like a really bad impersonation by an American to sound British (think Dick Van Dyke in "Mary Poppins.").
I should note that the Cockney dialect has become a general term associated with the British middle to lower class. However, technically it's dialect comes from the South(mainly working-class London and surrounding areas).
Now, it's the Irish-Gypsy(a.k.a Pikey)accent that I find the most challenging to understand (think Bradd Pitt in "Snatch")!LOL!
Do you even have trouble understanding the upper class British accent, such as in "Downton Abbey?"