Barter
I knew a black market butcher in Shepherd's Market. Fenced a lot of things in the Aldwych Underground station. But you sort of have to know who to ask. And it's usually the person you least want to talk to. I've seen some deals going down in the Lord Clyde, mostly for ration books. Now, if you see any of the Piccadilly commandos wandering around. Those girls will know where to fence a diamond.
But if I were you, I'm not sure I'd carry it on me when I was asking about it.