ANO article 26 authorises a G-reg flown with an ICAO (non-JAA) IR to be flown under IFR but only outside controlled airspace.
So, you could fly such a G-reg worldwide IFR in Class F or Class G.
This is somewhat useful in the UK where there is a lot of Class G and you can fly IFR in it without ATC clearance or indeed any radio contact.
Much of France for example is Class E which is controlled airspace for IFR, so this is no good.
Some countries (don't know which; Germany?) prohibit IFR in Class G and have no Class F.