What is the naming convention for European airways?
ATS routes in Europe are designated just like the rest of the world. ICAO publishes guidelines for the designation of ATS routes in Annex 11, which are adopted by almost all countries.

A route will consist of a possible prefix (see below) a letter (see below) and a basic designator (a number between 1-999).

The three types of prefix are:

K (Kopter) to indicate a low-level helicopter route

U to indicate an upper route, available only in upper airspace

or S to indicate a route only used by supersonic aircraft (not very common).

The letters can be:

A, B, G, R for regional routes which are not RNAV routes

L, M, N, P for regional RNAV routes

H, J, V, W for non-regional non-RNAV routes

Q, T, Y, Z for non-regional RNAV routes

In addition, states may add the letters F or G to the end of a route to indicate that the route is in class F or G airspace respectively. To my knowledge, this is not very common (and most ATS routes are in controlled airspace anyway).

Examples:

KY80 ("Kopter Yankee 8-0)

UN872 ("Upper November 8-7-2)

T54 ("Tango 54")