Technically, yes, USA-based websites can index and rank in the UK (or any other country). For instance, if you search for the brand name of an American-only company, chances are it will rank highly in the US, UK, and elsewhere in the world. But outside of a few exceptions like branded terms or some informational terms, it is really hard for the same page to rank in multiple countries and perform well. This is because so many types of searches have geo-specific relevancy (i.e. if you are searching for a product to buy, you'll more often than not want to buy from a company in the same country as you because shipping and other logistics will be easier to handle). So, if you are really wanting to get your website performing highly in the UK search results, it would be worth getting the UK TLD and building out a UK website (along with, more than likely, a UK arm of your business).