There are a couple of small record shops inside Wood Street Market in Walthamstow which I doubt are well known because they don't have much presence on the internet or their own website. I visited them a couple of times and they are a huge mess with a lot of crap to wade through but rewarding if you are into heavy rock or jazz. Loads of cheap records but don't expect to find obscure music in there.
http://www.woodstreetindoormarket.co...-indoor-market