The FRTOL no longer allows you to be issued with an AGCS Certificate of Competence (C of C) whereas a person holding a valid AFISO licence or ATCO licence with valid ADV/ADI ratings can simply pay the fee and apply for the issue of the C of C. To provide AGCS you must be in possession of this C of C, which must be signed by the licencee of the radio station which you wish to operate; there is no requirement for 'on the job' training or competency checking as there is for FISO or ATC Licences. To carry out on the job training for the 'validation' of an AFISO Licence, you must hold a C of C.
To obtain a C of C, you must pass written and practical exams with an examiner who is authorised by the CAA to carry out these exams. There are no formal training courses as far as I'm aware, but the exam is based on CAP452.