Barclays have a US branch.
Barclay's have an office in the states, or at least did a couple of years ago.....Detroit or Chicago, can't remember which. Point is that funds from your local UK branch can be wired to a Barclay's dollar account in the states ( set up in the UK) and from there into any US bank account. The more money you move the better the exchange rate, try playing the game with several UK banks as most have US branches. It pays to shop around and you may get a better deal by offering to open a new UK account for the transfer rather than going with what's in situ.