Fuji, I suspect it is arguable.
On the one hand, if a PPL can let a non PPL handle the controls provided the PPL still has controls and can override, why would that not also apply in IMC? It might come down to a court having to rule whether the extra risks of loss of control in IMC make it somewhere on the scale between injudicious (not necessarily illegal?) and reckless endangerment (which is illegal).
The ANO says this:
“Flight crew licence requirement – Exception for dual flying training
53.—(1) A person may act as pilot of an aircraft of which the flight crew required to be carried by or under this Order is not more than one pilot for the purpose of becoming qualified for the grant or renewal of a pilot’s licence or the inclusion or variation of any rating in a pilot’s licence within the United Kingdom, the Channel Islands and the Isle of Man, without being the holder of an appropriate licence granted or rendered valid under this Order, if the conditions in paragraph (2) are satisfied.
(2) The conditions referred to in paragraph (1) are that—
(a) the aircraft is not flying for the purpose of commercial air transport, public transport or aerial work other than aerial work which consists of the giving of instruction in flying or the conducting of flying tests;
(b) the person acts in accordance with instructions given by another person holding a pilot’s licence granted under this Order or a JAA licence, in each case being a licence which includes a flight instructor rating, a flying instructor’s rating or an assistant flying instructor’s rating entitling that other person to give instruction in flying the type of aircraft being flown;” [snip]
The fact that this is the only exclusion to “act as pilot” (not PIC, just a pilot) suggests that no other exclusion is permitted, so only a IR-instructor can let a non-IR licensed other person fly in IMC unless a court rules it is not dangerous. IMHO. But I am not a lawyer – where are they when you need one?
Chris N.