Where can I exchange British pound notes for dollars?
If you're a Barclays or Bank of America customer, you can use your debit/ATM card to withdraw funds at the other's machines with no extra fees. https://locators.bankofamerica.com/international.html
ATM is always your best bet. Bringing pounds and exchanging to USD at any store front in the airport or anywhere else ends up with lots of fees and or a terrible exchange rate.
The op can correct me, but I believe the OP has taken his UK trip and is trying to get rid of his GBP back here in the states, rather than obtain them. The atm here wont help at all.
Travelex is the easiest, but you may want to ask around if anyone is going over that wants money on hand prior to their arrival. Check the online exchange rate calculators for a fair price. I've done this a few times years ago and it works out well for everyone.
Good point. Might of misread... Obviously an ATM won't work. You could probably donate the money.