Lockdown

The Earth went into a lockdown at some point in the year 2020, in response to the COVID-19 pandemic. (PROSE: The Invasion of London, Flower Power, The Edge of Glory) The purpose of it was to "slow the spread" of the infection. (HOMEVID: Goo!)

Lucy Wilson was stuck at home with her family during the lockdown. She found the isolation difficult. (PROSE: Copy/Paste, Sweet Revenge) During the same lockdown, Francine Jones went to stay with Sylvia Noble and Wilfred Mott as her house had been stuck mid-renovation. (WC: A Message From Francine Jones)

Also that year, Gloucester was briefly placed under lockdown by the Judoon, using a zonal enforcement field, as they searched for the Doctor. (TV: Fugitive the Judoon)

The British government believed that lockdown might be the product of some sinister conspiracy, and asked P.R.O.B.E. and other such organisations to investigate the matter. Giles thought these conspiracy theories as espoused by the government were ridiculous, but investigated anyway and ended up uncovering an unrelated plot by the CIA to cover up dangerous experiments with a device that created rifts in reality. Though it had not been instigated by the CIA, lockdown helped downplay the public awareness of the strange events which resulted from this. (HOMEVID: Goo!)

Other types of lockdown
The Celestial Omnibus had a "lockdown" feature for emergencies, which would seal the bus up indefinitely. (AUDIO: The Iris Wildthyme Appreciation Society)

Behind the scenes
The lockdown mentioned in Sweet Revenge was a timely reference to the real life lockdown that took place due to the.