The difference between CAS and un-CAS is the difference between a known and unknown traffic environment. With the complex and busy airspace of the UK and the competing demands of civil (commercial and GA) and military traffic, airports that do not reach a certain level of traffic/passenger numbers are unable to justify the restrictions on other airspace users of CAS, and the system we have has developed to allow co-existence.
In the interests of brevity, I'll relate your last question to the top level of radar service available outside CAS, the Radar Advisory Service. Under a RAS, the pilot is expected to comply with all instructions as they would under radar control (level, heading, speed etc.) and the controller assumes responsibility for separating the aircraft from other traffic under their control, and for attempting to provide separation from unknown traffic. As this is an 'advisory' service, the pilot can elect not to follow instructions, and then becomes responsible for their own separation.
You can read about it all
here in the UK MATS part 1. Section 1, Chapter 5 relates to radar services.