Source : Webster's Revised Unabridged Dictionary (1913)

Britannia \Bri*tan"ni*a\, n. [From L. Britannia Great Britain.]
   A white-metal alloy of tin, antimony, bismuth, copper, etc.
   It somewhat resembles silver, and is used for table ware.
   Called also {Britannia metal}.

Source : WordNet®

Britannia metal
     n : an alloy similar to pewter