I've just finished reading an article written by Katherine Eustace entitled Britannia: Some High Points in the History of the Iconography on British Coinage. It traces the history of the Britannia designs and suggests that the designs evolution was greatly inspired by significant social, political and economic factors. It's an interesting read with a handful of plates.  
So I'm wondering - which of the Britannia designs is your favourite?  
Britannia first appears in Roman coins.  
  
With the restoration of the monarchy Britannia appears first in a medal (1660) 
 
and then in a number of farthing patterns.