konstantin,
Re. the 1200 AGL, in general terms, ICAO wise, the whole of the volume of any class of controlled airspace is available for aircraft operating in that airspace.
It is aircraft in Class G that are required to maintain a buffer (in this case below) controlled airspace, and the ICAO buffer is expressed in meters, but is rounded off to 700' ( not 500' or 1000').
Thus, you will find the base of a CTA is often 1200 AGL, so that an aircraft can fly 500' AGL minimum and still be 700' clear below the CTA. This is not limited to US.
I cannot give you an answer for the origin of the 700' where there is an IF approach, when I get some time, and as a matter of curiosity, I must search it out. It's probably buried in TERPS design procedures, as a first guess.
Tootle pip!!