TheOddOne said, "We're talking about the UK version of Class 'A' airspace here, where traffic is all IFR. Separation is maintained between aircraft by ATC, whether or not the pilots can see out (OK, there's Special VFR available, but that isn't applicable in the discussion here)."
I fully understand that these aircraft are on an IFR flight plan but when they are in Visual Meteorological Conditions (VMC) it's the pilots who must look out for other aircraft and not run into them. Time and again I've had to remind controllers not to panic when I'm in VMC.