I’m astonished. In civilised countries this service is provided routinely at any bank branch. Hell, their are lots of US banks with branches in London. The only thing that stops one of them from issuing you a cashier’s check drawn on a sterling account in their own London branch is sheer bloody-mindedness.
This smells like bullshit to me. There are many ways to transfer funds between a US and UK institution, and vice versa. What’s the nature of the recipient you are trying to send funds to?