The files contain a wide range of UFO-related documents covering the years 1978–2002. So if you want to find out more about lights in the sky over Waterloo Bridge, near misses by pilots, crop circles - and what the UK government thought of it all - this is the place to start. The files are in PDF format.
here is the link to UK government national archives
http://ufos.nationalarchives.gov.uk/