Currency is Pounds Sterling GBP. Euros are sometimes accepted as well particularly in larger stores, but it is best to assume otherwise. Note that although Bank of England notes are accepted all over the United Kingdom, you may have trouble with using Northern Irish and Scottish notes in England due to shop staff being unfamiliar with them.
Credit cards are accepted in most shops and restaurants. Visa and Mastercard are the most widely accepted, though debit cards with the Maestro logo are also taken. American Express cards are accepted in fewer establishments, but most restaurants will accept it. Credit cards with a Chip and PIN have become nearly compulsory. Credit card agreements mostly require merchants to accept cards with a swipe and signature, however, it is wise to carry enough cash in case the retailer does not comply.
One thing to keep in mind is that due to credit card surcharges, some establishments and shops will only allow cards to be used including debit cards over £5 or 10
