Jaggers calls Wemmick the most cunning imposter in all of London because Jaggers disagrees with what Pip said that Wemmick is a kind-hearted person. Because the Wemmick that Jaggers knew is different from what Pip described him to be. The Wemmick that Jaggers knew is different. The Wemmick that he knew can often be such a calculating individual in his business dealings. That is why Jaggers jokingly referred to him as the most cunning imposter in all of London.