Although I generally agree with all the posters above that using your debit card in an ATM is one of the most reliable and low-cost ways to get foreign currency abroad, I just want to highlight that if you're only getting a small amount of cash, the fees can actually be higher than using an exchange counter.

ATMs will generally give you a relatively good exchange rate, but charge a fixed fee. If you're going to withdraw only, say, 40 pounds, a total fee (ATM owner's fee + your bank's fee) of 4 pounds is 10% of the total - worse than the exchange rate you'd get even from airport money changers who usually offer rates 5-10% worse than the bank rates.

This is mostly an issue in developing countries where due to how cheap things are, you may only want to change the equivalent of 20 or 30 USD. In this ATM fees can quickly add up to more than 10%.

So for large transactions of 200+ GBP, EUR, etc. it's a no-brainer to use an ATM. But going and taking out 20 EUR at a time here and there a few times on your trip will quickly rack up ATM fees far beyond what you'd lose exchanging at a poor rate.