You may even fly under IFR rules in VMC condition without any instrument rating at all...
That is still possible on a UK National licence but has never been legal on a JAA or EASA licence!
What does it mean?
It means what it says, flight in accordance with the Instrument Flight Rules so provided the flight was legal, any flight where you complied with the rules counts.
Assuming you flew under UK Rules: Rules 20 and 32 applied
Choice of VFR or IFR
20 (1) Subject to paragraph (2) an aircraft shall always be flown in accordance with the Visual
Flight Rules or the Instrument Flight Rules.
Instrument Flight Rules
32 (1) For flights within controlled airspace rules 33, 34, 35, 36 and 37 shall be the
Instrument Flight Rules.
(2) For flights outside controlled airspace rules 33 and 34 shall be the Instrument Flight
Rules.
If you held a JAA licence you should be aware of:
JAR–FCL 1.175 Circumstances in which
an IR(A) is required
(a) The holder of a pilot licence (A) shall
not act in any capacity as a pilot of an aeroplane
under Instrument Flight Rules (IFR), except as a
pilot undergoing skill testing or dual training,
unless the holder has an instrument rating
(IR(A)) appropriate to the category of aircraft
issued in accordance with JAR–FCL.
(b) In JAA Member States where national
legislation requires flight in accordance with IFR
under specified circumstances (e.g. at night),
the holder of a pilot licence may fly under IFR,
provided that pilot holds a qualification
appropriate to the circumstances, airspace and
flight conditions in which the flight is conducted.
National qualifications permitting pilots to fly in
accordance with IFR other than in VMC without
being the holder of a valid IR(A) shall be
restricted to use of the airspace of the State of
licence issue only.
IFR has nothing to do with Hoods or Weather!