James,
Much as those of us who build British style firearms would love to have the appropriate proof marks on the barrels, we don't and it's not because it wouldn't be relatively easy to have a die maker build a set of stamps. I suspect in the UK applying such marks to barrels would be a felony. In my opinion it would be, at best, unethical for us to do it over here. Collectors use those marks to verify the authenticity of pieces, to date them, to determine if they are safe to shoot, etc. I only ask, do you really want to apply antique London or Birmingham proof to a contemporary American made piece?
Tom