You might want to approach your bank over this.
International inter-account transfers generally get a better exchange rate than cash forex, at lower fees. Obviously, there's a catch - you need [access to] an account in each currency.
Most British majors offer USD accounts to UK residents. You won't earn interest and you'll pay charges, but they can be worth a look.
It's useful to do some research online - few retail bank staff specialise in this area. Visit your bank's homepage and head for the international section.
