2015 Britannia Premium Gold Proof 6 Coin Set Box Coa - Mintage 138 Changing Faces of Britain. The collection includes the £100 Gold Proof Coin weighing one ounce of pure Gold, the £50 Gold Proof Coin weighing one half ounce of pure gold, the £25 Gold Proof Coin weighing one quarter of an ounce of pure Gold, the £10 Gold Proof Coin weighing one tenth of an ounce of pure Gold, £1 Gold Proof Coin weighing one twentieth of an ounce of pure Gold and for the first time the 50p Gold Proof Coin weighing one fortieth of an ounce of pure gold.