What about that "magical" currency conversion?
This is kind of a fallback solution if your shop is not multi-currency. The conversion is done by Google Shopping itself. If the products in your shop are listed in USD and you choose to target customers in United Kingdom, Google will do the math and convert the price from USD to GBP. People will see GBP in the ads, and when they click to go to your shop, they pay in USD. Most would not mind that, as long as the conversion is right (we can always trust Google can’t we?).