Corporation of the City of London
Group
In addressing the 'square mile' concept Londonist have provided a potted history of the City of London.
Worth noting that there are at least two plaques erected by the Corporation outside the City of London, both for William Blake: South Molton Street and SE1. Conversely Londonist points out that there is only one official blue plaque in the City, and it's not blue.
See other memorials in this area