Please remember that JAR-FCL is written for all of Europe. The UK is unique in that it allows non-IR (or IMC Ratng) holders pilots to fly IFR. In fact it is a requirement at night!
While there may be a difference, if one carefully reads JAR-FCL, one will see exactly how the two elements of IFR flying are to be logged and it makes little difference in the end;
In the JAR-FCL logbook, Operational Condition Time is recorded in column 9. This is flight time undertaken at night (night column) or under instrument flight rules (IFR column). Yes it does say IFR and not IMC so log IFR time and not just IMC time.
However, The remarks column is to be used to record among other things "instrument flight time" and JAR-FCL makes it clear that "A pilot may log as instrument flight time only that time during which he operates the aircraft solely by reference to instruments, under actual or simulated instrument flight conditions."
Thus the pilot who does lots of IFR flying in VMC (at night in the UK for example) will not have any instrument flight time recorded in the remarks column. The instructor teaching IMC while the flight retmains VMC would be in the same category. However, If the flight is in IMC the the pilot in command must log instrument flight time because after all, who else in the aircraft is qualified to act as crew member in IMC? Anything less would to be making a false record of the flight i.e. trying to say that the flight was VMC with the student under the hood when actually it was in cloud.
To put it another way, if I read an instructor's logbook and it records an IMC course training flight with a student but does not record any instrument time then from that entry I am being told that the flight remained in VMC even if it was under IFR.
Regards,
DFC