The United Kingdom's official currency is the pound sterling (£). Euros are not legal tender in the UK and are generally not accepted in shops, restaurants, or other establishments.
Some businesses in high-tourist areas (e.g., major department stores, airports, hotels) may accept euros as a courtesy, but they are not obligated to do so. If they do, they typically apply their own exchange rate, which is often unfavorable, and may charge a fee.
Exchange your euros for pounds sterling before making purchases. Use banks, currency exchange bureaus, ATMs (with a debit card), or contactless payments in GBP for the best rates and convenience.