One reason GA people want some sort of procedure is to take advantage of technical or noise-reduction progress elsewhere, approved by a reputable authority, but nit yet approved by CAA and too expensive for a small outfit to duplicate all the testing cost etc.
One example: Pawnee for glider towing in Sweden, modified to take Volvo 6-cylinder engine and 1:3 reduction gear big propellor - about 60 percent improvement in fuel consummption and much lower noise.
Total success in Sweden, approved years ago on experimental basis by their Authority, collecting more practical experience daily etc.
No chance of Piper applying for UK certification. Volvo helped the Swedes, but won't do anything official for fear of legal liability issues - a few aviation conversions are too much trouble/too little revenue for a car company.
We can't even get one started in the UK to demonstrate its potential.