After setting up an online gallery to sell my artwork I was thinking of approaching photography galleries in London to sell the artwork. Now the problem is I have never approached a "real world" gallery before. I would really appreciate so advice from individuals who have experience in this area.

Should I be focusing on galleries with a specific niche? What sort of commission rates is the gallery likely to want? I guess I am after so advice to build a base of knowledge that I can work with and negotiate with.