access to USD in the UK directly from USD account?

Thanks a lot for the replies - will look into all of them, looks like I will need to purchase from an exchange as a best option.

Voyager2002: Pretty poverty stricken countries with conflict, so typically ATMs there are disbursing the local currency (IF connected to the network and actually functioning) while USD (often newer notes etc.) works as an informal currency as well (and are sometimes preferred because local currencies can fluctuate and be unreliable). Not to mention, in some places withdrawing equivalent of 1000 USD would result in a bag of money basically because the currency notes are of such low value and so small. Since bank cards and cheques can't be relied on as these kind of economies are basically cash based it's pretty essential to turn up for the first time in these places with USD cash (and to have on hand for general emergencies). Typically it is then possible to set up a local bank account in USD through my work for part of my salary and withdraw USD bills from a cashier old skool style but depends on the place, and doesn't work on first arrival. Usually, I would withdraw from the US, or have dollars from closing down my last local dollar account, but I don't have either this time around....so, I need dollars in hand before flying.

Thanks - I'm just trying to work out if it's possible in the UK to get dollars without converting to GBP and back to USD - I know how to deal with travel/expatism otherwise. I'm arriving in a place where you can not rely on digital banking or ATMS so landing without hard currency (and pounds are not as useful or accepted as dollars in this regard) is basically insane. If you ask a lot of foreigners or wealthy people living in places like yemen, south sudan, somalia, afghanistan most will tell you they all carry USD cash - and, of course, on first arrival will be carrying some regardless of subsequent arrangements while living there