This is going to sounds terribly ignorant, but I never worried about this stuff.
VFR outside CAS, one can fly anywhere. The quad levels are UK only anyway, abroad one flies the semi levels, and if flying an ATC directed level or route (rare in the UK but not rare abroad) then one flies the appropriate flight level, and usually the higher the better.
VFR in CAS, one does what one is told. ATC don't seem to care about this, and assign you any level that suits them, even if it is completely "against the rules".
IFR outside CAS, UK, one can fly anywhere but one tries to fly the correct levels.
IFR in CAS one does what one is told.
Interesting about the transition layer. Never knew about this!