I recent filed a FOI act request to the UK Ministry of Defence, and had a response dated 18 July 2013.
"Thank you for your email of 20 June 2013 asking how the MOD responds to unidentified aerial traffic entering UK airspace and detected in UK airspace; the effectiveness of our air defence to unknown quantities and unknown events that may and have occurred, and details of such occurrences between 2000 and 2012. We have now completed a search for the information you requested and I can confirm that the MOD holds some information within the scope of your request. I am treating your correspondence as a request for information under the Freedom of Information Act 2000.
RAF Typhoon Quick Reaction Alert (QRA) aircraft are held at continuous readiness at RAF Coningsby and RAF Leuchars. QRA aircraft scramble to identify any aircraft approaching or in NATO-monitored airspace or National airspace without prior approval or not having identified themselves and which cannot be identified by any other means i.e. the aircraft is not talking to civilian or military Air Traffic Control, has not filed a flight plan, and is not transmitting a recognisable secondary surveillance radar code.
The following table gives the number of days within each year that QRA launches of fighter aircraft has occurred. Details of launches prior to September 2006 are no longer retained by the department and we have therefore started the table at September 2006 from which point the requested information is held by the department.
Year Number of days QRA launched
* Records held from Sep 2006"
Whether these encounters are what would be terms UFOs is subjective but never the less one can assume a response to such detections would provoke a reaction by the MOD