https://www.nationalarchives.gov.uk/help-with-your-research/research-guides/ufos/
https://www.archives.gov/research/military/air-force/ufos
https://www.livescience.com/uk-ufo-reports-soon-released.html
British Naval Air Force documents, fact sheets, interesting reference to Roswell incident from page 235 titled „What Roswell incident was NOT„:
https://www.physics.smu.edu/pseudo/UFOs/dod174.pdf
The better quality of this document (of US Air Force particularly about Roswell incident):
The book by Dave Clarke issued by the National Archive – fully devoded to the UFO phenomenon, versatile probe of investigation many main and auxiliary topics as the main phenomenon radiates on other issues, disciplines
https://www.nationalarchives.gov.uk/documents/the-ufo-files-extract.pdf
This is the very useful document by David Clarke PhD from National Archives about how to search from the masses of documents for those that may contain traces of what particularly may interest you
https://www.nationalarchives.gov.uk/documents/ufo-highlights-guide-2013.pdf
Example of yearly report from all UFO incidents reported to the body collecting date day after day: