Just had a quick look at NZ rules and airspace classifications, which are of course very different to the UK.
NZ
Controlled airspace A, C, D. Appears organised like the US for Class C and D. Class A is the same as UK airways, but appears to only be high level for commercial jet traffic.
There are 'Airways' in class G that are defined routes that guarantee terrain clearance. It is mandatory to be receiving traffic information if operating under IFR, but appears to rely on self separation for collision avoidance.
UK
Airways are almost always class A and can be quite low level.
Class C is used like the the US Class A to ensure all aircraft are under positive control at 'jet' levels.
Class G, there is absolutely no requirement to talk to anyone or receive any service IFR or VFR, VMC or IMC.
There are no (or almost no) Class G advisory routes, and the Class F routes are in very remote areas. So effectively, 'all' airways are controlled.
most GA IFR flights are on random routes where altitudes for sufficient terrain clearance are established by the pilot during flight planning.