But why FL180? There is no terrain higher than 5000 feet. FL 80 would seem more realistic.
FL 180 for the whole of Europe perhaps?
Contacted,
The Transition Altitude for the whole of the US and Canada is 18,000', even though the vast majority of the terrain in both countries is below 3000'. It just makes life simpler.
As others have pointed out, when below 18,000, you are given new Altimeter Settings as appropriate (and they are NEVER referred to as QNH).