DFC, As a pilot who receives a FIS from Farnborough many days per week I believe that their allocation of individual squawks to aircraft receiving only a FIS is a matter of pragmatism. In addition to the aircraft to whom they are providing a FIS they are also handling their own IFR inbounds/outbounds plus those from the Farnborough clutch.
Due to the number of aircraft in the open FIR, plus the gliders from Lasham it is often necessary for the EGLF controllers to negotiate with an aircraft receiving a FIS and agree a temporary limitation on their operation (eg “not above 2400ft”, “route to the east of the field”, etc) in order for the EGLF inbound/outbound to be handled efficiently. Without individually identifiable aircraft this would not be possible.