I use a Starling account to transfer £ to my US account in $. Quick and easy, online and a better rate thanm I used to get from Moneycorp (part of Barclays).
Incidentally getting cash out in euroland on my Starling card also gives me a good exchange rate .. but always avoid the Eurobanks who want to give you a "guaranteed" price in £, as it'll be at least 10% worse.