I think these coins if it was made public in the press and in the media would easy fetch £250 each , after all they are much much rarer than the mule 20p and ten times rarer than kew gardens 50ps which are actually quite common.
All it needs is for someone to leak this news to the daily mail and every chav in the county will be seaching their pockets looking for these coins (with no success, by the way!)