So far the CAA have been rather vague on this subject.
When I was in South Africa in the 1970s it was all very simple.
Transition altitude was 19000 feet and anything FL200 and above was controlled airspace, IFR only.
We should not conflate this question with the Quadrantal vs Semicircular question which is a separate can of worms. IMHO the sooner we harmonise with europe on this one the better.