The real value of a proofhouse, to the buyer of a used gun, comes in those various proofmarks....
....The gun trade may set the rules, but it's your police and legal system that handle enforcement. Our gun trade also sets rules. However, those rules no longer apply once the gun goes onto the second hand market. That's where our system--or lack thereof--provides less peace of mind than yours.
The proof marks in your context are for historical curiosity or correctness. The key to proofing in the UK is the mandatory registration process, and in America would only give peace of mind to those who have a reason to track and regulate. Individual peace of mind for those who want to proof their guns has never been restricted by anyone, but that's probably not going to be considered enough.