ICAO G is no sep of course, so you must have a difference registered.
The ICAO Annexes and PANS specify the minimum level of service to be provided to IFR and VFR flights in different classes of airspace. They do not preclude the application of separation in other circumstances.
The class G airspace in which a UK-style Approach Control Service or Radar Advisory Service is provided could not be class A-E because there is no obligation for IFR flights to participate. Whether it could be class F is open to a little more debate, but since a service cannot
always be provided to IFR flights thaat wish to participate, I think it has to be class G.