I can give you a little more information on the UK (and European) ATS Route (airway) naming conventions. The naming conventions are laid down by ICAO for each region and in the EUR region, are:
For non-RNAV ATS Routes that are part of the region's basic route structure, the prefix letters A, B, G, R may be used.
For RNAV ATS Routes that are part of the region's basic route structure, the prefix letters L, M, N, P may be used.
Each country/region also usually has a number of ATS routes which are not considered to be part of the basic route structure, such as overload routes, by-pass routes and short link routes for internal ATC use which have different prefix letters:
For non-RNAV ATS Routes that are not part of the region's basic route structure, the prefix letters H, J, V, W may be used.
For RNAV ATS Routes that are not part of the region's basic route structure, the prefix letters Q, T, Y, Z may be used.
As Ernie mentioned, the addition of the prefix letter 'U' indicates a high level ATS Route. In the UK, this is a route above FL245 but this does vary from country to country.
So, the example for the UK that you quoted - UN590 - is an RNAV Upper ATS Route (above FL245) which is part of the region's basic route structure.
Pete
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