Of course! I have even personally sub contracted in the past for European companies. When you do business in the U.S. you must pay taxes and report to each state in which you do business. I have been able to work out a loop hole in the past for marketing services across state lines due to the fact that I don't sell actual goods or even digital goods - only consultancy work. But as a UK company you would need to register in the U.S. (or have a contractor/partner) do so and go from there.