Forcing country selection and currency

Hi apologies if you have seen similar post which I placed under "ecommerce" but this is more relevant to us UK users actually:

when ex-UK customers are using the STORE, how can you ensure they select say US$ as their currency prior to going to checkout?

We have lots of orders that come in under currency=GBR although the Customer address (as entered during checkout) is US, and we have the dropdown "select your currency". But many customers don't see / can't be bothered.

And as you know, once a transaction is done by a Customer (Sales Order/Cash Sale), their default currency can no longer be changed. So we end up manually settin up duplicate Customers and Orders with the right currency.