The City of London Cemetery have published the areas they tend to re-use. See here.
Memorials will be either be reused with the tombstone being turned around so that the original inscription will be on the back, or if ‘dilapidated’ the tombstone will be destroyed, but a permanent record of the inscription will be kept.
Objections must be sent before 31 May 2013 to:
The Superintendent Registrar
City of London Cemetery,
London E12 5DQ