I don't know the ins and outs of this but I happen to have a JAR-FCL compliant logbook at home (which I use as a sort of backup logbook) and in the 'instructions for use' it says this:
Column 11: Pilot function time: enter flight time as pilot in command (PIC), student pilot in command (SPIC) and pilot in command under supervision (PICUS) as PIC. All time recorded as SPIC or PICUS must be countersigned by the aircraft commander/flight instructor in the Remarks column. Instructor time should be recorded as appropriate and also entered as PIC.
So if I understand correctly a dual instructional flight is logged by the student as SPIC time (with dual time going in the adjacent column) which goes in the PIC column of the logbook. The instructor then also logs 'instruction' time as well as PIC time.