The Secretary of State for Defence, shortened to Defence Secretary and sometimes nicknamed the Defence Minister, was the MP in charge of the United Kingdom's Ministry of Defence.
The Defence Minister during the Axos affair had sent Horatio Chinn, a man he loathed, to investigate UNIT and ensure they were following MoD guidelines. When Britain had the chance to have a monopoly on Axonite, he granted Chinn the powers to arrest UNIT and was left politically embarrassed when this was leaked to the United Nations. (TV: The Claws of Axos)
In 1999, Jennifer Hamilton was Defence Secretary. She colluded with the Prime Minister's Voracian scheme and was murdered when she tried to back out. (PROSE: Millennium Shock)
In 2005, a panicked Defence Minister called MI6 to report the activation codes for the nuclear arsenal had been wiped. (COMIC: The Flood)
An unnamed male Defence Secretary was part of the attempt to cut a deal with the the 456 (TV: Children of Earth: Day Three, Day Four) and ultimately ordered the British Army to kidnap children and deliver them to the aliens. (TV: Children of Earth: Day Five)