If you're living in the UK and you're a USC, you get to file taxes both places. But you'll really only pay one place. UK gives you a tax-free allowance of 11,850 pounds and then it's a progressive tax for anything above that. So you pay it, then claim it against anything you owe the IRS. If you don't earn at least the 11,850 pounds, you'll not pay to the UK (but you do have to report it). You'll pay Uncle Sugar instead.