I now at last have the answer !! ( And typically my sources who asked the CAA got it wrong !!
If flying a G reg on an ICAO ( non JAR ) IR , the UK ANO prohibits IFR in controlled airspece. Ref: ANO article 26 (4) (a) (ii) in the case of a pilots licence, to act as pilot of any aircraft flying in controlled airspace in circumstances requiring compliance with Instrument Flight Rules
Since everything NOT prohibited IS permitted , this means you can fly a G reg on an FAA PPL/IR worldwide VFR in class B,C,D,E,F,G and worldwide IFR in class F,G.
To be practical about it , IFR in class F/G is not useful outside the UK since most airspace in which IFR is authorised (or one can get an IFR routing in ) is at least Class E. Its useable in the UK but only partially : you could fly IFR to say Biggin Hill ( Class G ) but not to Bournmouth (Class D ) This is why getting an IMC Rating is desirable since it extends IFR to Class D , however you then also need a UK PPL , UK Medical etc