I also wondered about Elizabeth I, but I think there may be more than one explanation why she was omitted in this stamp series. For one, she has probably already been the central figure on at least one series of stamps in the past decades, and then also this series seems to be focused on those royal figures who did not survive their time in the Tower. Although I wonder given the appearance of more details about the fate of the Two Princes, if they belong in this current stamp series? They may well have survived after all, it seems? As Elizabeth I of course did.