Currency exchange before departing or after arrival??

You'll get a better rate when you arrive at Heathrow.

And be sure you don't "exchange US dollars". Use the ATM at Heathrow.

The ATM will give me a better exchange rate?? Great!! Thank you!

Natalie,

I'd suggest taking a small amount of "travel funds" with you, perhaps £50 or so. That way you'll be able to buy meals and pay for rail or Taxi fare from the airport. Once you get settled in the U.K., find the nearest ATM and "top up" your cash.

I usually take the largest withdrawals allowed, so that I can minimize service fees. Leave enough money for daily expenses and store the surplus in a Money Belt.

Note that your travel funds accessed from an ATM must be in a chequing account with a four number PIN.

Happy travels!

I'm sorry if I'm stating the obvious, but...you don't exchange dollars at an atm. You just use it as an atm, just like at home, and you get the local currency from your home checking account. (I had an image of someone trying to feed $20 into an atm like it would make change.)