A KNS80 would be no good for European airways. You often get a DCT or "own nav" to a waypoint which - even if it is a VOR/DME - is way too far away for reception.
ATC simply assume you can navigate to point X no matter where it is.
You could tell them you can't, I suppose. I had an IFR GPS failure once, and got them to give me vectors / VOR nav. But this isn't normal procedure at all.