I'm a flea-market nut, but there's nowhere in London I'd particularly recommend. The biggest market is Brick Lane (sunday am), but decent cameras are now almost non-existent. In the past, Bermondsey Market (Fri am) had some decent stalls, but I've not been since they redeveloped most of the market.
However, if your free day is Thursday, check out Greenwich covered market. There's a guy called Tim who usually has a lot of folders, plate cameras etc; I noticed he had a Barnack last week. His prices are good - less than eBay. I bought a good, working Autocord from him for £45.