in an intercompany scenario between 2 plants you do not have 2 customers (what is a shipping customer? do you mean ship-to? ) you just have a one vendor and one customer.
only if your goods flow is vica versa, then you create a vendor and a customer for each plant.
e.g. plant A is vendor A and customer A, plant B is vendor B and customer B.
if A orders at B, then A orders at vendor B, and B ships to customer A
if B wants material from A, the B orders at vendor A and A ships to customer B.
Customer A has a sales org data view for the sales org used by B to sell the materials
Customer B has a sales org data view for the sales org used by A to sell the materials
In this sales org data view you enter the currency in which you bill the customer.
So your US plant bills in USD the goods that got shipped to UK.
And your UK company enters the bill. The currency will default from vendor master purchasing data view and should be USD (same currency that you use to order). Now you have the paper invoice and you enter it in MIRO, if the currency defaults to USD then enter the bill right away, if it defaults in a different currency than on the paper, then you have to enter the currency along with the value yourself.