Sold in cases of 500 1oz coins.
From 2013 the silver coins have a millesimal fineness of .999 (99.9% silver). From 1997-2012 the silver coins have a millesimal fineness of 958 (95.8% or Britannia silver).
Silver Britannias have been released each year beginning in 1997, when a silver proof set was offered. In 1997 and in all subsequent even-numbered years the reverse design has depicted a standing Britannia figure. Beginning in 1999 and continuing in odd-numbered years, a series of alternate, non-repeating depictions of Britannia have replaced the standing figure on the reverse.