What do the UK regulations say about logging "IFR" time? In the US, there is no such thing in a log -- only actual or simulated Instrument Time ("sole reference to instruments" as Pitdown noted).
Under US rules, in general, IFR and VFR are mutually exclusive. You can fly under one or the other. When flying under VFR, you CANNOT legally fly actual IMC, so the only way to log Instrument Time is Simulated Instrument Time with a Safety Pilot on board. When flying solo under IFR, you cannot log actual Instrument Time when you are VMC, because you still have "see and avoid" responsibility.
Bottom line is: Either actual IFR + IMC, or have a Safety Pilot on board.