"R" stands for "rules"; and "MC" stands for "meteorological conditions."
When in VMC, you can legally fly under VFR or IFR. When in IMC, you can legally fly only under IFR.
In the LHR-JFK example, it is unlikely that the airplane (a commercial airliner assumed) could fly under VFR (below 18,000') to JFK. If in VMC, a VFR return to LHR would be smarter.