There are four sizes of gold English coins : sovereign is the most commonly used (around 8 g., diameter 22 mm), half-sovereign (around 4 g., diameter 19 mm), two sovereigns (around 16 g.) and five sovereigns (around 40 g.). There are sovereigns in the effigy of George III, George IV, William IV, Victoria, George V and Elizabeth II. The reverse is a coat of arms or St George slaying the dragon.
You were a lot to ask me the value of a sovereign. These coins are to be sure quoted on the Stock Exchange, you can easily find their current value by clicking in the following links :
Current price of the sovereign (sovereign = around 8 g., diameter 22mm)
Current price of the half-sovereign (half-sovereign = around 4 g., diameter 19 mm)
The price of the sovereign does not stop varying according to the course of the Stock Exchange ; for example, the value of a sovereign was around 140 euros in 2008, whereas the value of a sovereign was lower than 99 euros in 2004.