IO's point is very important for the UK controllers who don't also fly.
A foreign pilot files a flightplan, receives and ack with the plan unchanged and then on starup at LKPR gets told 'Cleared to EGHH via the TANGO1, departure 124.25 squawk 2654' - He has had a pretty strong mindset that he is cleared through all of the controlled airspace in his flight plan. Yes he should know he is going to dip out of CAS. But having a transfer to London Information being the main clue to him he no longer has clearance into the next bit of CAS seems a bit risky and inviting the pilot to blunder into CAS if between him and Info they don't twig that he has his FMS programmed to plung him back into CAS in 20 miles and needs to be getting Info to get his new joining clearance.
HD
I have never flown in Africa so have no idea if it is just like the UK - or like the Wild West where everyone needs to make it up on the fly (but I have certainly heard a number of Wild West stories!)
In the US there are lots of airways all class E and the little bits between are Class G - However, if you are on an IFR plan, even if your DCT takes you through a bit of OCAS they don't cancel the remainder of your clearance and make you to call for rejoin - (From memory if it is a small bit of OCAS they don't even tell you) - AND you can't fly IFR without an ATC IFR Flight Plan and the difference between the radar service you receive in E or G is pretty marginal (either you are in coverage or not and you never get separation from VFR only advisories)