In the United Kingdom, only coins valued 1 pound Sterling, 2 pounds, and 5 pounds Sterling are legal tender in unlimited amounts throughout the territory of the United Kingdom. In accordance with the Coinage Act 1971, gold sovereigns are also legal tender for any amount. The face values of sovereigns are 50p, �1, �2 and �5; their actual value-being linked to the price of gold-is much higher. The United Kingdom legislation that introduced the 1 pound coin left no United Kingdom-wide legal tender banknote.
Currently, 20 pence pieces, 25 pence coins (although many retail outlets do not recognise or accept this) and 50 pence pieces are legal tender in amounts up to 10 pounds; 5 pence pieces and 10 pence pieces are legal tender in amounts up to 5 pounds; and 1 penny pieces and 2 pence pieces are legal tender in amounts up to 20 pence.
Coins and banknotes do not need to be "legal tender" in order to be used as money to buy and perform other transactions for which money is intended. For example, British banknotes issued by various institutions circulate in the United Kingdom without being legal tender in all the jurisdictions of the United Kingdom.
Easy C&P from here:
http://en.wikipedia.org/wiki/Legal_tender