Confirming or denying if a licence is held in a specific name is not a breach of data protection according to the CAA. Why does an instructor sign your log book for a dual training flight? So that an examiner can call the CAA and identify that this person is a bona fide instructor by stating his name number and asking the same question!
I would imagine the correct way to go (although probably extreme) is get the police to look into it
No! If you have concerns about a person flying without a licence they should be made to Aviation Regulation Enforcement, part of the CAA not the police as suggested. ARE which is staffed by ex policemen has immediate access to all information held on the individual. So if you are really concerned that's another way to confirm if a person holds a licence.
Head of Aviation Regulation Enforcement
CAA Secretary and Legal Adviser’s Department
CAA House Room K504
45-59 Kingsway, London WC2B 6TE.
Telephone: 020 7453 6193