I wonder how many midairs are caused by autopilots?
Almost all midairs are of aircraft not on autopilot, the only two I can think of (in the last 5 years or so), where both aircraft were on autopilot for most/all the run up to the accident, were the Brazilian head on between a CAT and a corporate jet and the überlingen crash between two jets - in the second case both on TCAS and under radar control.
The UK ones are mostly GA GA in or near the pattern, doing aerobatics, or low level maneuvering GA vs fast jet. I doubt any were on autopilot other than maybe G-EYES (the Practice ILS calibrator at Coventry).
Midairs are most likely on a gin clear day at relatively low level with people flying by looking out the window (which is not in any way to see not looking is safer!). But they also are thankfully rair.