You don't always get to choose the reg number from a list. Sometimes they let you choose, other times they choose for you. It depends on the dealer.
Out of the three brand new cars I've bought over the years, the Fiat dealer sent me letter regarding my Punto's reg number, with my Civic the dealer phoned me up about the reg number, and with my second Jazz the dealer let me choose from a list on the day I ordered the car (it was not the same dealer who supplied my Civic).