Most of the lower class characters have cockney accents in the theater version, too. I think it has to do with the show originating in London and that accent being associated with the lower class in popular culture. I loved Helena Bonham Carter as usual and enjoyed Sasha Baron Cohen. i knew they'd do a great job with those characters.
Laurie
The musical was originaly french. It was translated into english, and got so popular that people don't even know it was french first.