If the flight (as far as we are aware) is going to be wholly within CAS then the clearance limit will be the destination airfield. If we know the flight will be going out of CAS when leaving the CICTZ then the boundary will be the clearance limit.
I think this is an idyllic description which does not relate to reality.
I could file a route from say Split in Croatia, to Goodwood, of which the last 50nm may be OCAS.
My departure clearance at Split will be "cleared to EGHR...".
The departure tower has no way of knowing if any part of the filed route will be OCAS. Well, it might do if you fly IFR from Bournemouth to Goodwood, but that is a daft route
Coming back to my "international pilot" example... this is important. It doesn't matter whether you have a JAA IR or an FAA IR or a Mongolian IR. You
will get caught by this weird practice.
The only pilots who won't get caught will be those who did their
UK JAA IR on the Bournemouth / Oxford / Cambridge run
and who then buy their own planes, and fly for real at levels which are potentially OCAS (i.e. did not go to the airlines).