In the Class G context, nobody can "control" traffic.
Therefore, is there a legal obstacle to a FISO providing an "information" service for the purposes of scheduling traffic to the IAF?
In the USA, they deal with this by the IAF being in Class E, which is CAS for IFR and positive ATC control can thus be exercised.
But in the UK, ATC could not control the traffic anyway, even if the billing issues were resolved. All they could provide is a standard radar or procedural service, which any pilot could disobey legally (if he advises ATC of the fact).
And if the UK IAF was in CAS, say some existing Class D, we would then be back to the old problem of ATC sometimes refusing entry into CAS, thus making the airfield sometimes inaccessible to IFR traffic.