Not using correct currency for location

I have a site where the default currency is GBP, so all products have their price set in GBP but the store shows everything in USD.

However, if the customer has an address does not have a country set, or if the country is set as UK then the prices on the site display in GBP, even though it is set to display prices in USD for the UK.

Here’s the settings screens:
General: http://bit.ly/2Fw1oFQ
Location: http://bit.ly/2ZYrQRW
Checkout: http://bit.ly/2N47Fwy

Do you know why this might be happening?

Just to clarify, we want it so that prices are always displayed in USD for customers anywhere in the world.

Thanks,

John