A lot of comments on this thread about CAS being an issue here however if the incident happened as a lot of reports have said between two aircraft inbound to Coventry then it isn't really the main issue as both aircraft were hopefully in contact with ATC as they were landing at the field.
CAS is all about creating a known traffic area so that ATC can deconflict transit traffic against inbound or outbound traffic. So CAS in my view isn't the main factor here as an incident like this could quite as easily happen between an ILS inbound and circuit/visual joining traffic in CAS as it could at an ATC airport in class G.
The only difference between this situation occuring just outside an ATZ in class G and at an airport in CAS is that you could say that there is no legal requirement for pilots outside CAS airspace to comply with ATC instructions however In my experience this has never been an issue.