Whatever the AAIB outcome, we can conclude that two aircraft colliding in Class G is very likely to have had an element of poor lookout as a causal factor.
I don't see much evidence for that - other than tradition.
As has been said here many times before, assuming straight trajectories, a target on a genuine collision course will be a
stationary point in your field of view, and unless contrast is working in your favour you won't spot it until too late.
All those who 'nearly hit and would have done had I not taken evasive action' were PROB99 not on an accurate collision track with you in the first place.
The Mk1 eyeball is a cornerstone of aviation tradition (along with epaulettes, stopwatches, circular slide rules) but in reality it is about as good as a chocolate teapot. It works if the target is not on a straight trajectory so it probably works OK in gliding, but in GA most people fly straight lines most of the time.
If one plane is flying the ILS, accurately, and perhaps doing an ILS survey (the wreckage movie on sky.com shows the gear UP and normally you would have the gear down at 2-3D if you were going to land) and somebody flies in from the side at say 100kt, you won't spot him.
No doubt we won't find out - rumours and leaks aside - why the SE traffic was where it was until the AAIB report comes out a year from now, which is a great pity IMHO.
The great protector is the big sky and incredibly low traffic density (compared to say a road). That's why midairs happen at low levels, generally 1000ft or below.