In broadcasting and radio communications, a call sign (also known as a call name or call letters—and historically as a call signal—or abbreviated as a call) is a unique identifier for a transmitter station.
Source: Wikipedia
This is a tool to check call signs published by Ofcom (Ofcom is the UK’s communications regulator). This tools searches the given call sign in the database published by Ofcom on 15th of January, 2024. If the searched call sign is not found in the Ofcom database, that does not guarantee that the call sign is still available. It may have been assigned after the aforementioned date.