Pizza Hut. It's actually quite good here. I'm disappointed by the lack of breadsticks, though. I tried to explain to my boyfriend what they were, and it finally clicked when I googled the American Pizza Hut menu and showed him what they were.
I can't wait to taste those again when I go back to the US.
There is Frankie and Benny's, but I'm not entirely sure if that's American. They claim to be American, but nothing on the menu is really American at all. I ordered "American pancakes" expecting big round pancakes with maple syrup, but I got basically what are Scotch pancakes. Tiny and only 4 of them, for £4! I could have gotten 8 for 60 p at Sainsbury's.