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, AUDIO: Best Year Ever) The purpose of it was to "slow the spread" of the infection. (HOMEVID: Goo!)

After resolving the fractured timelines caused by the crash landing of the Doctor's TARDIS and restoring the true 2020, the Eighth Doctor, Liv Chenka, Helen Sinclair, Tania Bell and Andy Davidson found themselves in the midst of the lockdown. (AUDIO: The Keys of Baker Street) Unable to leave Earth in the TARDIS until the end of the year for fear of destabilising time again, the Doctor and his companions lived through the lockdown in 107 Baker Street whilst Andy Davidson kept working, due to being an essential worker. Eventually the Doctor became so frustrated by being unable to intervene in the pandemic due to time's fragility he retreated to the TARDIS interior for the remainder of the year. (AUDIO: Best Year Ever)

Lucy Wilson was stuck at home with her family during the lockdown. She found the isolation difficult. (PROSE: Copy/Paste, Sweet Revenge)

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)

Code 23 was the lockdown procedure for the Cheviot Hills site of the Vault. (PROSE: A Worthy Successor)

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