Hi there,
suggest you have another look ar CAP452 the Air Ground Radio Operators Guide and see the chapters on Air Ground from CAP413. As you say A/G is a pretty basic service which can be difficult for some A/G operators to apply at some places when they get busy. FISO has a certain "control" up to the Holding Point of the active runway. Clearances can only be relayed from ATC verbatim. This is very much a generalisation.
Hope it helps.
