What is the data on the % of CAS busts v. how many were in contact with that ATC unit?
My impression is that most potentially serious busts are done as major nav errors, by non-radio flights (by non-radio I mean not talking to a radar unit; e.g. talking to London Info is going to be of no help)
As for "unknown traffic", if every such traffic called up some unit, the whole system would collapse. The UK ATS system is not geared up for providing any kind of services outside CAS to everybody who wants it - not by a factor of ten times. Far too many little planes flying about. It might just work in OVC007 weather when there is very little GA traffic anyway.