Hi AA,
In the London City Control Zone we are vectoring traffic at 2000ft downwind for runway 10 only 1 nm inside the boundary of CAS, both North and South of London City Airport. The CTR is only 4 nm either side of the extended centrelines and we are required to provide 3 nm radar separation from traffic established on final. Very often unknown traffic comes right up to the very edge of the CTR in anticipation of a clearance through, before actually contacting Thames Radar. All we can do is pass traffic information to the IFR traffic, there is little room for manoeuvre. If we turn the IFR traffic away we are in danger of losing separation from other IFR traffic in the sequence. As 'almost professional' states, we would much rather be talking to VFR aircraft operating on the environs of controlled airspace as they then become known traffic.
To answer your questions. Does it make any difference if you are known or unknown traffic operating outside CAS in this context? Well accordingly to the book - no. Traffic operating outside CAS is deemed to be separated from traffic operating within CAS, as 'almost professional' has said. Having said that, if I know your intentions and have verified your altitude it makes for a much safer situation.
The lowest level we routinely descend IFR traffic to is indeed 500ft above the base of controlled airspace. The boundary of controlled and uncontrolled airspace must be set somewhere, and to the East of EGLC/EGKB the base is 2500ft, so you can quite legally fly at 2499ft underneath when I am descending to 3000ft. I will sometimes pass traffic information on unknown VFR aircraft to my commercial IFR flights, as they are likely to receive TCAS alerts anyway if the VFR traffic is transponding. The same situation applies to Heathrow Approach descending to 3000ft in the White Waltham area. We cannot practically apply standard separation between known and unknown traffic.
Whilst I am on the subject, the use of GPS has meant that pilots tend to cut corners, and therefore pass very close to the Eastern edge of the London City Control Zone. On Easterly operations and especially on hot days when climb performance is less than optimal, aircraft departing from runway 10 barely make their SID levels by the edge of CAS, mainly due to a very limited track distance from take off. They could come unnecessarily close to unknown traffic just shaving the CTR, and not talking to Thames Radar. Again I would much prefer to have identified the VFR traffic and warned it of the situation.