What is the importance of "Grey London" in the novel, A Conjuring of Light?

Grey London is the world as the reader knows it. It is set in nineenth century England, when King George IV rules over the British Empire. This world has forgotten about magic but communicates with Red London occasionally through letters delivered by Kell to the Five Points tavern and to the king.

The only known magician from Grey London in Red London is Lila Bard, a pickpocket. The magical doors to Grey London that can be accessed by tokens were closed long ago; only Antari, like Kell and Lila, can travel to and from Grey London because of this.