Government Communications Headquarters
GCHQ is not subject to the Freedom of Information Act and therefore will not process such requests. Under Regulation 4 of the Environmental Information Regulations (EIR), environmental information relevant to the functions of the Security Services, which does not adversely affect national security can be requested.

CESG is the Information Assurance division of GCHQ and the UK Government's National Technical Authority for Information Assurance.  CSEG runs GovCertUK, the Computer Emergency Response Team (CERT) for the UK Government.[1]