Does the US postal service sell money orders denominated in pound sterling? What about Western Union?
Try American Express. It’s where I get my Pound Sterling Traveler Checks (oops Cheques)
Most banks would be able to make out a cheque in pounds for you.
My experience is that only larger branches of banks are willing to write traveler’s or cashier’s check in a foreign currency. Local branches can tell you where you need to go.