In the UK instrument time is described in LASORS Section A:
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.
LASORS suggests that instrument flight time should go in the remarks column (yeah, I know most logbooks have a dedicated column). Flight time under IFR is a required item (in the same way flight at night is).
"Time during which he operates the aircraft" suggests in a vague way that only the PF can log instrument time.
MC.