While I agree with the general sentiment, I don't believe any country defines IFR as 'by sole reference to the instruments'. The unusual bit for the UK is allowing non-IR rated pilots to fly IFR and defining the IFRs in such a loose way that for operations below the TA, in Class G and in sight of the surface (I.E. the conditions most PPLs fly in most of the time) IFR and VFR are pretty much indistinguishable.