The Britannia is a British gold coin manufactured by The Royal Mint and contains one troy ounce of fine gold (31.1035g).
As this coin is technically legal tender in the UK, it has the benefit for British investors of being exempt from Capital Gains Tax (as well as VAT).
The Britannia is now manufactured from 24 carat gold (i.e. pure gold) unlike other British minted bullion coins. This is a particularly attractive product for British investors, being a conveniently tradable size, whilst also being CGT exempt and being available at a low margin over the gold price