What's a working class atmosphere when it's at home? Pearly Queens striding the market streets, a greater percentage of cheekie chappies, more Elizabeth Duke hoops than usual?
TBH, I'm not sure Brixton market's changed in class all that much. Short of a couple of additions (an upmarket, understocked deli and a quality pizza joint) that could loosely be considered 'posh, the market's largely of the same feel (to me at least) and make-up as it was 30 years ago. Many of the same traders or relatives trading as well. Certainly the underwear stalls ain't got suddenly more classy.
It's smaller, less busy and more rundown for sure, but that's not a class issue for me. East Street may feel a little more 'bustly' at times, but I'm mystified by your perception of it feeling more 'working class'...