Thanks ATCO Two and Almost Professional. I find it scary that traffic outside CAS is 'deemed' to be separated from traffic inside.
It is inconsistent that you provide 3nm/1000ft separation minima to aircraft inside your known traffic environment, but accept and use 501 feet or 1nm (ATCO Two)/2nm (Almost Professional) minma from unknown traffic (possibly IFR) outside CAS. It is certainly inconsistent, but is it unsafe? To my knowledge, there have been no accidents attributed to such loss of separation. I wonder how often you ATCOs have had to change your plan and vector CAS traffic away from an unknown tracking erratically close to your boundary?
I take your points concerning talking to the ATSU. It works for units like Essex and Luton, but I doubt that the London TMA controllers would welcome calls from GA traffic.
Is there a training point here? Especially in IMC, should pilots routing close to CAS maintain a minimum distance of say 2nm from the boundary? I'm not suggesting a legal requirement, but having heard your comments, it would be good airmanship.