So, controllers - I'm on a long leg, I'm IMC qualified, I may well be flying quadrantal rule (or can easily adjust my height to do so), and receiving a basic service. VFR/IFR hasn't been discussed, no flight plan has been filed -I'm just flying in open FIR. And there's a big bank of cloud in front of me - so I elect to fly through it, as there's no reason not to. What do you want me to tell you? Serious question, as I really cannot recall ever being taught this and am pretty certain that there's nothing in CAP 413 about it either.
In response to "traffic passing left to right two miles ahead no height information" I replied "thanks, we're IMC just now". The controller continued to call traffic, and we continued to ignore any that wasn't explicitly at our level, on the grounds that at least 99.999% of it was going to be beneath the cloud base, not messing around in the clouds just for the hell of it as we were. (This was an IMCr navex, real cloud was much more educative than wearing a hood.)