BRITANNIA COINS
Britannia coins are British bullion coins issued by the Royal Mint in gold since 1987 and in silver since 1997.
Britannia gold coins contain one troy ounce of gold and have a face value of £100. Gold Britannias also are issued in fractional sizes of one-half, one-quarter, and one-tenth of a troy ounce and with face values of £50, £25, and £10 respectively.
Britannia silver coins contain one troy ounce of silver and have a face value of £2. Silver Britannias also are issued in fractional sizes of one-half, one-quarter, and one-tenth of a troy ounce and with face values of £1, 50p, and 20p respectively.

GOLD BRITANNIA

From 2013 the gold coins have a millesimal fineness of .9999 (or 24 carat gold). Until 2012 the gold coins have a millesimal fineness of .917 (91.7% or 22 carat gold) with the non-gold component being copper until 1989 and silver from 1990.
2013 Gold Britannia specifications (gold content, not total weight)
One ounce: diameter 38.61 mm, (31.104 g)
1987-2012 Gold Britannia specifications (gold content, not total weight)
One ounce: diameter 32.69 mm, (31.104 g)
Half ounce: diameter 27.00 mm, (15.552 g)
Quarter ounce: diameter 22.00 mm,(7.776 g)
Tenth ounce: diameter 16.50 mm, (3.110 g)

SILVER BRITANNIA

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.

Silver Britannia (U.K.)
Face Value:	£2 GBP
Mass:	32.454 g (incorporating 1 Troy ounce of fine Silver
Composition:	1997-2012 95.8% Ag 2013 - 99.9% Ag
Diameter:	1997-2012 40.00mm 2013 - 38.61mm
Edge:	Reeded
Years of Minting:	1997–present
Reverse
Design:	Britannia
Design Date:	2009
Obverse
Design:	Queen Elizabeth II
Design Date:	2009

This is the Britannia Silver Coin I buy 22 April 2013 :) 
- Britannia 2 Pounds 1oz Silver Coin 2012