I might be able to help you, depending what sort of a frame you're after, and what your idea of "fairly low" is.

I've got a couple of gas-pipe road frames (both probably 70's) in about a 56, which you could have for £10. I also have a really nice frame that I suspect of being 531, pretty lugs, etc, but it's more like 52 cm. I'm in Southampton, but I come to London all the time. Drop me a PM if you want to discuss further.