To clarify the UK position, valid until the revised airspace classifications (ICAO) come on stream.
In Class G airspace: daytime, VFR or IFR; night-time IFR (as has been said, VMC at night is tolerated but there's no allowance for VFR)
In Class D airspace: daytime, VFR, SVFR or IFR; night-time SVFR or IFR
In Class A airspace: daytime and night-time, SVFR or IFR
