Greater London was established in 1965 as an administrative unit covering the London metropolis. It was not defined as a county but had a two-tier structure, with the lower tier being the London boroughs.
Following the abolition of the Greater London Council (GLC) in 1986 the London boroughs became single-tier authorities, but Greater London was still widely recognised, especially for statistical and mapping purposes. In 2000 however a two-tier structure was re-established when the new Greater London Authority adopted responsibility for a range of citywide policy areas.
There are 32 actual London boroughs, with a status similar to metropolitan districts, and also the City of London, which is a City Corporation and has a number of additional roles. London boroughs are subdivided into electoral wards.
You can find a full listing of London borough names and codes on the Open Geography portal. The names and codes are in the UK local authority districts downloadable file. Within this file the London boroughs have codes beginning with E09. For more information on the coding, please refer to the Code History Database (CHD).