foghorn
Just to take the corrections a stage further, CAAFU has not existed for over a decade now.
An "IRE" as defined in JAR-FCL is qualified to conduct IR issue tests i.e. a CAA FE.
Renewals are conducted by either a CRE/IR (SPA) or a TRE (MPA).
The differences between the IR and IMC rating are given in ANO Schedule 8.
The holder of an IR may fly in controlled airspace in circumstances which require compliance with IFR. Additional privileges include a lower SVFR minima and no restriction on fly out of sight of the surface.
The privilege of the IMC rating, which is only valid in the State of Issue, extends the privileges of the holder of a pilot licence to fly in Class D and E controlled airspace in circumstances that require compliance with IFR. It also reduces the SVFR visibility minima to 3 Km, and permits flight out of sight of the surface. Compliance with IFR in F and G airspace is not mentioned in the IMC rating as these privileges are not excluded from the basic UK licence.
JAR-FCL 1.175 states that the holder of a JAA licence may include National ratings e.g. IMC for use in the State of Licence Issue. Schedule 8 states that the IMC rating extends the privileges of a JAA licence to include flight in Class D and E airspace in compliance with IFR, but makes no mention of Class F or G airspace!
Instrument Flight Rules have nothing to do with the weather!
Having an IR is only part of the equation, the aircraft must also be suitably "qualified" to exercise the privileges of the IR.