The Ministry of Defence's UFO desk and its public hotline ran from 1950 to 2009. On closure the surviving files — 52,000+ pages across the AIR, DEFE and PREM record groups — were transferred to The National Archives at Kew in successive tranches between 2008 and 2017. All are digitised and free to view via Discovery. Reusable under the Open Government Licence v3. Source: TNA UFO Reports.
Hand-picked highlights from TNA's UFO topic page plus deep-links into Discovery for the full collection. The press release PDF below is local; everything else loads on Discovery itself.