The Farnborough situation is not unique in the UK, there are many airfields both with and without radar with iaps in class G airspace.
In the Farnborough case, offering ATC services to transit traffic mitigates, to some extent, the lack of controlled airspace by creating a 'known traffic environment; most people who fly through the iap are on one or other of the Farnborough frequencies. If Farnborough did not have ATC of any kind, it would be a strictly VFR airfield as there are no holding or letdown aids associated with the iaps ie nothing to get you onto the ILS, (apart from comms. fail procedures which cannot be used without radar) and in any case with the proximity of class A airspace only 5 miles away, you couldn't have a purely 'pilot interpreted' letdown due to the necessary 'buffer' required.
I was the person responsible for getting Farnborough iaps approved so I like to think I know what I'm talking about.