UK
The British Ministry of Defence (MoD) ran an official UFO desk from 1950 until December 2009, when the unit was closed and its files committed to The National Archives (TNA) at Kew. The release programme that followed — coordinated by TNA's UFO project — declassified roughly 60,000 pages across 208 file series, covering 1947–2009 reports from civilians, RAF stations, NATO allies, and air-traffic-control units.
What MoD did and did not do (verbatim, MoD statement, Dec 2009)
"In over 50 years, no reported UFO sighting has revealed any evidence of a potential threat to the United Kingdom. The MoD has no opinion on the existence or otherwise of extraterrestrial life. However, in the absence of any evidence of a threat, the Department does not investigate sightings of UFOs in order to identify them."
Key file series
DEFE 24/1948 — Rendlesham Forest, December 1980. The most-cited file in the corpus.
AIR 2/19061, AIR 16/1199, AIR 20/9994 — RAF reports and intercept logs.
DEFE 31 — Defence Intelligence Staff files; includes the 2000 Condign report (commissioned by DIS "to provide an intelligence assessment of UAP").
HO 305 — Home Office records, including witness correspondence forwarded by police.
The Condign Report (2000)
A 460-page classified DIS report, declassified May 2006 under FOIA request. Its conclusion: "That UAP exist is indisputable. Credited with the ability to display a number of reported phenomena, no evidence exists to suggest that the phenomena seen are hostile or under any type of control, other than that of natural physical forces." Condign attributes the bulk of UAP observation to "buoyant atmospheric plasmas" — a hypothesis that remains contested.
The TNA UFO project
Between 2008 and 2013, TNA released the MoD files in tranches, each accompanied by an explanatory podcast (Dr David Clarke, TNA's UFO consultant) and a public-friendly micro-site. The micro-site at nationalarchives.gov.uk/ufos/ remains the most-cited national-archives UAP portal globally.