Actually, there is no concept or definition of "primary nav" in the ANO.
The ANO specifies equipment to be
carried. It does not specify equipment to be
used.
The only time equipment
usage may be specified is on AOC ops where you fly IAW a company ops manual.
The whole business of "primary nav" (GPS being good / no good etc) comes from various pompous leaflets emanating from the CAA, or the pompous GASIL/GASCO organs which are written to appear as if they are written by the CAA
In truth, a private pilot is free to navigate with any, or any combination of, navaids, and is 100% legit to do so. Above MSA or below MSA
Above FL095, Europe, IFR, the carriage of an IFR GPS is the only acceptable (GA context) means of compliance with the BRNAV equipment carriage requirement.