Its the least sophisticated English accent. If the orcs were American they'd sound southern. Orcs with a London accent would remind me of the smart gremlin in Gremlins 2, it would be weird. They should sound stupid.
@bargainben: I don't know... Southern Orcs would sound pretty stupid. I could see mobster New York Orcs better, personally.
We have the privilege of having several unintelligent sounding accents to choose from. New Jersey, Tennessee, Creole, take your pick. I'm kinda bummed one of those hasn't been indelibly attached to a race of fantasy monsters. Like why cant gnomes sound like New Orleans shrimp boat captains by default, that would rock.