I guess if your No Plate is not age related, on a car like the Sportster, you'd get away with it.
Just tell the plod you thought it was how old the plate was, not the car.
Seriously, the ones you see are propably 'flouting the law'.
For a proper answer have a look on here:
http://www.pistonheads.com/gassing/forum.asp?f=10&h=0