Definitely don’t use your Bank, you’ll get a rubbish rate and most likely be hit with other charges.
You should use an international money transfer specialist to sort it for you. I used these guys Hawk FX, following a recommendation from a mate who bought a property in Greece. They were awesome, making sure I got the most amount of dollars for my pounds will zero hassle.
Top tip, if you are not in an immediate rush to move the money to the US make sure you let them know, it increases the number of options available to you on getting more dollars for your pounds!