United Kingdom uses the British pound (symbol: £), code GBP.
Banknotes come in denominations of 100, 50, 20, 10, 5 and 1 pound. Pound is subdivided into 100 pennys. Coins come in denominations of 2 and 1 pound and 50, 20, 10, 5, 2 and 1 penny.
You'll get the best exchange using an ATM after you arrive - ideally at the airport itself. Do not use exchange booths (Forex, Travelex etc.) as they have lousy exchange rates. That being said, it's wise to have at least some cash before-hand in case of emergency (ATM not working etc.).
Coins can be hard or simply impossible to change back to euro when you leave a country. Before leaving, spend them (on postcards, a newspaper, or food or drink for the train ride), change them into banknotes, or give them away.
If you have some pounds left at the end of your trip, change it to euro at the airport or simply spend it at the airport before you fly home. That way you won't have to deal with pound currency when you come back home.