1) On a private flight (i.e. non-AOC, so there is no company procedures manual approved by the national CAA) you are allowed to use any means of navigation you wish, including the proverbial
tuna sandwich
2) In Euro airspace, the carriage of a BRNAV approved navigation receiver is mandatory. In the context of currently manufactured civilian avionics this can be met only with a) INS or b) an IFR GPS.
3) ATC watch you like a hawk and expect precision performance. When they say "DCT XYZ" and 20 seconds later you are still pointing 10 degrees away, they are onto you, even though at that point your lateral track error is probably only a mile or two. Evidently they watch the computer-extrapolated track...
So, in Europe, "total RNAV" is a done deal and the only question is whether you are doing it with an FMS (with DME/DME or GPS corrections) or directly with a GPS. It's obviously very easy and effortless to fly this way.
If you lose RNAV capability you need to advise ATC, and they will either give you vectors, or give you some VOR-able route.
BRNAV does not (currently) apply below FL095, but if you want to see the feasibility of developing valid routes around Eurocontrol-land below FL095, download a copy of
this and have a little play, setting the max level to FL095, and see how far you get before you run out of juice
Actually it works in some places, like France.
PRNAV is something else....