Well I suppose part of it comes down to the slightly odd definition of IFR we have in the UK. If it's CAVOK outside and someone calls up a LARS in the Open FIR claiming to be IFR at FL40 or whatever then the controller is going to know that "IFR" in this sense means that the aircraft just happens to be flying 1000ft above the nearest object within 5NM of track and is flying at the correct level for track.
What is more important to the controller outside of controlled airspace is whether one is VMC or IMC....and claiming to be IMC when one is VMC is just odd.