because historically the possession of a FRTOL used to allow people to operate an airfield A/G station
Very historical; The only licence that entitled the holder to operate a Ground Radio Station was the Flight Radio Telephony Operator's (General) Licence which predates the CAA by a number of years. Probably last issued in the 1960s. If you held a FRTOL(Restricted) you were exempt the exams to obtain a Radio Operator's Certificate of Competence (Restricted) which still has to be endorrsed by the ground radio station lincense holder before you can exercise the privileges of that certificate.
There is quite a lot of information re licensing on the
CAA Website