Logbook question - where to enter IMC time etc

If you undertake all or part of a flight in accordance with IFR then that IFR time goes into the IFR column irrespective of flight conditions. The portion of the flight that is in actual IMC goes additionally into the IF column.

But in the UK you could undertake most of your flights in class G airspace IAW IFR without a specific IFR clearance, so in principle you could include all of those flights in clear VMC as IFR flights so long as you complied with rules 33 and 34.

Instrument practices conducted under VFR with a safety pilot count as IF but not IFR, although you could claim IFR if they are outside of controlled airspace.

Use your discretion

As above, any time you are flying IFR, whether it is in VMC or IMC then you log that in the IFR column. Time spent flying in IMC goes in the IF column (which is not required to be logged).

The important thing to remember is that you can be flying IFR in clouds or in CAVOK conditions, but any time you are flying in IMC it MUST be IFR (legally).

“Flying in accordance with the IFR” and “flying IFR” is the same thing.

What is IAW IFR?
When you say in accordance with IFR could you explain this one a little more?

Formerly Rule 20, now SERA 2005

SERA.2005 Compliance with the rules of the air
The operation of an aircraft either in flight, on the movement area of an aerodrome or at an operating site shall be in compliance with the general rules, the applicable local provisions and, in addition, when in flight, either with:
(a) the visual flight rules; or
(b) the instrument flight rules.

It is the set of rules you have chosen to fly in accordance with.

but any time you are flying in IMC it MUST be IFR (legally).

…except Special VFR.

The IFRs in the UK are rules 33-37 of the rules of the air (CAP393). Rules 35-37 only relate to flights under ATC control and in CAS.

Outside CAS it’s only necessary to comply with 33 and 34 for the flight to be considered as under IFR

Minimum height
33 (1) Subject to paragraphs (2) and (3), an aircraft shall not fly at a height of less than 1,000
feet above the highest obstacle within a distance of 5 nautical miles of the aircraft
unless:
(a) it is necessary for the aircraft to do so in order to take off or land;
(b) the aircraft flies on a route notified for the purposes of this rule;
(c) the aircraft has been otherwise authorised by the competent authority in relation
to the area over which the aircraft is flying; or
(d) the aircraft flies at an altitude not exceeding 3,000 feet above mean sea level and
remains clear of cloud and with the surface in sight and in a flight visibility of at
least 800 metres.
(2) The aircraft shall comply with rule 5.
(3) Paragraph (1) shall not apply to a helicopter that is air-taxiing or conducting
manoeuvres in accordance with rule 6(i).
Quadrantal rule and semi-circular rule
34 (1) Subject to paragraphs (2) and (3), an aircraft in level flight above 3,000 feet above
mean sea level or above the appropriate transition altitude, whichever is the higher,
shall be flown at a level appropriate to its magnetic track, in accordance with Table 1
or Table 2, as appropriate.
(2) For the purposes of paragraph (1), the level of flight shall be measured by an altimeter
set:
(a) in the case of a flight over the United Kingdom, to a pressure setting of 1013.2
hectopascals; or
(b) in the case of any other flight, according to the system published by the
competent authority in relation to the area over which the aircraft is flying.
(3) An aircraft may be flown at a level other than the level required by paragraph (1) if it flies:
(a) in conformity with instructions given by an air traffic control unit;
(b) in accordance with notified en-route holding patterns; or
(c) in accordance with holding procedures notified in relation to an aerodrome.
(4) For the purposes of this rule ‘transition altitude’ means the altitude which is notified
in relation to flights over notified areas

Is there any benefit or requirement to log explicitly IFR or Flight by sole reference to instruments, assuming you don’t plan to take any further qualifications or ratings that require it?

Part of the reason for asking is that my US instructor told me the only flights the FAA require to be logged are those which ensure you are legally valid – I.e. For qualified VFR pilots, just the BFR. I suspect most FAA pilots do log every flight but they don’t need to. IFR pilots would do so to demonstrate rolling IFR currency, but this doesn’t apply in EASA where an annual proficiency check is mandatory.

…except Special VFR.

Yes OK fair enough