Simply using logic: A vehicle is tested under the MOT criteria and fails. The owner says "Just kidding" let's go back to the old certificate which still has some days on it. Are the authorities, and perhaps more importantly the insurer, going to be happy when the vehicle within that period is involved in a collision or other incident ? Personally (and I have no links or quotes to resort to or rely on), I would have thought that as soon as you submit to an MOT test then the new results are what governs (and should), exactly as when a fail/rejected certificate/result is overruled by a new inspection which results in a sparkling pass. Again personally, I certainly would not want to proceed without a pass certificate (regardless of previous history, which with a pass or failure is irrelevant anyway) and simply hope all will be dandy. Indeed, in my opinion, if the authorities were to accept ignoring the more recent failure and go for the "best of two" in a bureaucratic mindset then I would add that to my growing list of farcical UK management procedures. (Personal memo: Check what the powers that be say next time an opportunity arises)