Surely the information is only as accurate as what the previous owner decided to confirm?
For example, I sold a car a few years back with 22,000 on the clock. A few months later, I received a letter from some official body (can't remember who) saying that the car had been sold by a dealer with 9,000 on the clock and could I confirm what mileage it had done when I sold it, with me being the previous owner.
Unfortunately at the time I was well stressed and didn't bother sending back the reply that it had done at least 22K when I sold it.
So what I am saying is that the National Mileage Register will only be accurate if the previous owners completed the details when asked, or on MOTs. Even if I MOT'd my car today with 30K on the clock, what is to stop me driving another 20K before the next MOT and selling it? The last recorded 'accurate' mileage would be the 30K from the last MOT, surely?