Nick here from Shopify.
With a UK Shopify payments account, it is not possible to to take USD directly as payment. This is because your Payments account is tied to GBP.
What you need is to show your prices in USD. We have docs and apps on adding this:
Show multiple currencies (Doc)
These will allow you to show prices on your store in USD. The checkout will still be in GBP.
Hope this helps!