Great question, some Trading Standards Offices have approved garage schemes -
Fife Trading Standards - Approved Garage Scheme
However, I don't think they are very widespread. The AA has approved autocentres
Car Servicing : services, MOT tests and repairs - The AA, but do we trust the AA now they are owned by venture capatilists. They seem to be diluting the brand and using it to sell anything.
I hope this helps
Tony