Flight levels on Airways were not allocated according to the Quadrantal Rule ie odd 500' levels were not used.Generally,even levels were used for Westbound flights,odd levels used for Eastbounds.There would ALWAYS be 1000' between aircraft when separation was provided by height alone.However,above FL 290,the minimum vertical separation was 2000',thus the first available level above this would have been FL310.Bear in mind however,that airways were etablished only in the lower airspace(FIR),up to FL245.Above that lay the UIR (Upper Information Region),which was all controlled Airspace (Special Rules Airspace to be exact),and there were not Airways,but Upper Air Routes.Things have now changed of course,with the advent of RMVSAs (Reduced Minimum Vertical Separation Areas),and 1000' separation is pretty much standard at all levels.
Last edited by ex82watcher; 6th Feb 2019 at 23:15.