And then there's this definition.
An airway is a legally defined corridor that connects one specified location to another at a specified altitude, along which an aircraft that meets the requirements of the airway may be flown. Airways are defined with segments within a specific altitude block, corridor width, and between fixed geographic coordinates for satellite navigation systems, or between ground-based radio transmitter navigational aids (navaids) (such as VORs or NDBs) or the intersection of specific radials of two navaids.
No mention of CAS.
I also found this bit of info as well.
In most land areas of the world, aircraft are required to fly airways between the departure and destination airports
Foxmoth. I think our Airways are more aligned to the FAA system with very little in common with the UK Airways. We shall have to agree to disagree. Plus we are getting away from the original discussion of un published let downs.