Just to clarify, the IMC is usable some places abroad, those British Dependant Territories listed in the ANO 2000 [Overseas Territories]....just to be pedantic
Anyone can fly IFR in uncontrolled airspace, but the 'standard' PPL has a restriction of 'in sight of the surface' and 'XYZ m from cloud' [can't remember what though]. The IMC rating removes these restrictions outside controlled airspace then has a little bit which states you can also use it in Class D airspace and below. It also reduces SVFR minima when transiting into Class A zones VFR [Channel Islands for example]. So with an IMC rating you can fly through the clouds, above the clouds and shoot an instrument approach into a Class D airport if you wish. You cannot fly in airways or in Class A airspace while flying in IMC.
The JAA IR in a G reg aircraft further removes the restictions on Class A airspace and because its an ICAO rating, it can be used abroad. You can also therefore enter airways.
Having a foreign IR, for example FAA IR in a G reg aircraft allows the holder to fly IFR in IMC outside all controlled airspace, and becasue many other european governments accept the british validation of a foreign IR this means you can fly outside CAS in IMC while abroard in a G reg aircraft. Not as useful as a JAR IR in a G reg, but more useful than an IMC rating abroard. You will also be issued an IMC rating by the CAA for free if you hold a foreign IR. Of course in a N reg aircraft with an FAA IR you are perfectly entitled to fly 'proper' IFR in any airspace in the UK / Europe / Aroard.
The confusing thing is filing an IFR flight plan whilst the holder of an IMC rating. For example the controllers don't know what your qualifications are, the other day I flew into Edinburgh and had to shoot the approach down to minimums to get in. No problem there as the CAA IMC minima is 'recommended'. However leaving Edinburgh I was cleared with the Grice 4 departure [or something] then Scottish control cleared me across Papa 600, which is class A airspace. [Not a problem in this case as I had a JAA IR'd pilot onboard, but had I been alone then theoretically I would have been illegal even though I hold an FAA IR], however had the tail number of the aircraft started with an N I would have been legal ??? Confusing or what !
It MAY be that in the future the CAA will state that a foreign IR holder can exercise the privilages of the IMC rating whilst in a G reg aircraft WITHOUT the need to have an IMC rating attached. If this is the case then the European IMC rating has just been born, as all European authorities I have contacted have replied that they will accept whatever the CAA accepts regarding foreign IR validation in a G reg aircraft.
Cheers
EA