<<This 'feature' does seem to be unique to the UK/CI airspace. Everywhere else I have ever flown seems to give a whole route clearance (even if it includes a DCT outside controlled airspace). >>
I was trained to ICAO standards in 1966 and that's how we did it and I employed that procedure whilst working in Africa and the UK. ATC cannot "clear" anything outside CAS so the pilot should be cleared to leave CAS at (position) and after flying through Indian Country he should then request clearance to rejoin.