It would seem that the old misunderstang still exists between VMC/IMC and VFR/IFR.
To clarify:
One may elect to fly under the Instrument Flight Rules (IFR) or under the Visual Flight Rules (VFR) provided that one is flying in Visual Meteorological Conditions(VMC). However, if one is flying in Instrument Meteorological Conditions (IMC) one has no option but to fly under the Instrument Flight Rules (IFR).
The specific rules relating to flight under IFR and VFR are, of course, laid down in CAP 393 - The Air Navigation Order and Rules of the Air. But you knew that, didn't you??