Yes I realise big jets are approved but this is of concern to GA which is myself and everybody on this forum, plus all bizjets.
Yes it is a concern to GA, however not for biz jets nor turboprops. Even a TBM850 or Mustang is PRNAV approved without any modification necessary, it is the OEM standard equipment. Dinosaur planes might have a problem, but they will have a much bigger problem with noise certification.
And there are not many IFR flights in SEP/MEP planes around europe, which in itself is a huge concern and makes their lobby extremely small. With airspace being the main constraint nowadays it needs to be managed more efficiently and for that we do need higher precision values like PBN RNP1 or even in the future 0.3 in terminal airspace to let procedure designers design procedures which fit more airplanes through any portion of the airspace system in a given amount of time.
I think the wording of the british AC is very precise and they use "where supported by fleet survey" to indicate that not all fleets do support those features (yet). Since the GA fleet in some parts does not support Advanced PBN stuff like DME/DME/IRU backup and all that time control stuff there will be exceptions for that fleet. Not to mention that a lot of that stuff is only applicable above FL195. Another of your concerns seems to be that "RNAV 1 (PRNAV) should form the baseline design standard." Well, it is about airspace design, not certification. And of course they have to use the experience made with PRNAV as that is currently only standard that supports something akin to RNAV 1. Moving into ICAO based PBN will mean a move away from the currently existing local and differing RNAV definitions like BRNAV and PRNAV.