Ask the bank here in the US about what is known as the exchange rate. There is what is known as a percentage of the money you change that you pay the bank to do the exchange, which is included in your bank money exchange. But the exchange rate is the monetary difference between Euros and Dollars. But you really want the English pound and not Euros if you are just going to London. Today as of September 16, 2017, you can buy one English pound for $1.25 American money, meaning that their money today costs 25cents more than one American Dollar. You know, it is just best to go to the American bank and purchase American Express traveler's checks or get an American Express card, and put the money on the American Express card and use the card to pay for something and use the change in cash as you go. Most of the big stores such as Harrod's will let you make a purchase but also get some cash back, much like American supermarkets do here. Do the same as you would here in the US, go to the big Harrod's store, make a small purchase, and get cash back from your American Express card in a small sum such a s 20 British pounds to spend in the small stores. You can then deposit the change into your American Express card through an ATM and then cash out in American dollars when you return to the US. Have a good trip!