Ask HN: Iphone 5s from USA work in UK?

Hey guys, I live in the UK, and it's much cheaper for me to buy an Iphone 5s from the US and take it back with me. I have actually just bought one, but haven't opened it yet. I got the no-contract AT&T version. Does anyone have any experience if the iPhone will work in the UK? Or should I return it? Cheers!