Yes, it was the HMRC. I did as much of it as I could, and I called them and waited half an hour on hold and got a guy who was bored, but not disdainful and talking down to me. I do have to put down how much I made in the States, but I won't be taxed because of the dual tax agreement, and I don't have to provide proof, either, which seems rather trusting of them.
If you make less than 6,000, do you get all of your tax back? Because if so, I'm due back about 650 pounds, which would be reaaally useful right about now.