I believe CAT Hamble lost some PA28's in mid airs around 1970.. don`t know whether they were in IMC though.
"No civilian aircraft in the UK operating in IMC outside controlled airspace has ever been involved in a mid-air collision."
You are quite specific about the scope of the thread and your stated requirement to support this statement. OK
But statistically it's an interesting case because to put it in to context surely you'd have to record and factor the number of occasions Controllers have issued avoiding action instructions to aircraft on an RAS where an actual risk of collision existed.