What's the difference in the US/UK version other than currency issues such as displaying in euros or pounds vs. dollars?
If you can combine them and allow the visitor to pick the country and switch from euros to pounds, I'd go with that because anything else will obviously be diluting your site. More importantly, if you do both a UK and US version of your shop and allow both to be indexed you'll need 100% different content for both or one will probably become supplemental.
If it's all the same stuff, I vote for less options, 1 site, 1 shop and worse case a separate checkout only (which isn't indexed) for UK or US.
There are carts available that handle international sales and shipping so it shouldn't be that hard of a problem to solve.