<font face="Verdana, Arial, Helvetica" size="2">If you hold a full FAA IR then your may exercise IMC priveldges in a G Reg aircraft.</font>
Not so Julian.
You may exercise the privileges of any ICAO licence for private flights on G-reg aircraft in the UK, but not within controlled airspace under IFR. That means that IFR flights in class D and E airspace are not permitted for FAA IR holders, but are permitted for CAA IMC rating holders.
It used to be the case (and probably still is) that the CAA would issue you with an IMC rating if you converted an FAA PPL/IR. But that's not the same thing as exercising the privileges without a conversion.