Just found this in Wikipedia
"Older British plates had white, grey or silver characters on a black background. This style of plate was phased out in 1972 and, until 2012, legal to be carried only on vehicles first registered before 1 January 1973. A vehicle which was first registered on or after 1 January 1973 shall be treated as if it was first registered before that date if it was constructed before 1 January 1973.[6] However, the Finance Bill 2014 and subsequent Finance Acts extended the Historic Vehicle class cut-off year from 1973 to 1974 and subsequently, a rolling forty years. This had the effect of linking eligibility to display old-style plates with 'Tax Exempt' vehicle status. It follows that the older style plates are now available for any vehicle constructed 40 or more years ago, provided that an application has been made to the DVLA to have the vehicle included in the historic vehicle class; it is so registered and is nil-rated for Vehicle Excise Duty."
