Sorry, Shane, what I've got at home is not the complete British standard but just an excerpt of it published by an English piping fittings manufacturer. The publication goes up to 8 inches, which is the maximum diameter he makes.
800 millimeters is approximately 34 inches, so your stub end doesn't fall under any American standard. ANSI B36 goes up to 24 inches.
Now, looking for the British standard, I've found an old Taylor Forge catalog describing the special fittings they manufacture, including stainless steel stub ends and backing flanges. It's important to point out that those fittings, from 26 to 36 inches, doesn't follow any published standard; they're made "under Taylor Forge internal standards". Now, being Taylor Forge a well known, highly respected manufacturer, we can safely take for granted what he says.
Having said all that, I'll tell you that the maximum underthickness tolerance for a 34 inches, stainless steel stub end facing is 20 thousanths of an inch, about 0.5 millimeters.
Does this informations help you in any manner?
Giovanni S. Crisi