ec155mech
Allocation of callsigns is coordinated worldwide by ICAO. They publish DOC 8585 "Designators for Aircraft Operating Agencies Aeronautical Authorities and Services" which specify approved callsigns for aeronautical use. Each ICAO Contracting State (ie country) is required to administer their own allocation system with allocations fed back to ICAO to ensure that duplication does not occur.
To use a callsign for a UK registered aircraft you will have to request allocation by CAA. It will not be agreed if it duplicates something already specified in Doc 8585.
My understanding also is that CAA will not accept applications for callsigns unless the applicant is an AOC holder.