Incidentally, what is the lowest that commercial traffic is likely to be above the base of CAS?
Above? If you live near Norwich, Coventry, EMA, Humberside or others, then "commercial traffic" (i.e 737's et al) is in the open FIR at whatever height they choose, along with the rest of us (as indeed they are entitled to be).
Do not mistake CAS with the routing of airliners.
DT