Welcome to WebmasterWorld, CY - good to have you here.
Since 80% of your site visitors are from the U.S., it doesn't sound like U.S. traffic is the issue. So I'm not certain what impact having a .com URL would have.
Do you think the ratio between the percentage of U.S. visitors to sales is related to your location in the U.K, or possibly because of currency conversion issues?
If it's possibly only currency conversion that's an issue, you could try having an alternate ordering/billing procedure for non-U.K. residents, and see if that will impact the number of orders, before taking more complicated measures such as setting up another domain, or going through the mechanics of arranging for country-specific fulfillment.
One route for international ordering and currency issues could be to set up something like Kagi [kagi.com] on a trial basis, for non-U.K. purchases, to see if there will be any improvement, since the currency issue would be eliminated, while shipping would still by local to you.
Based on a few months' analysis, you could then see if doing anything further would be warranted.