How are Americans treated in the uk (generally) ?

If your mother taught you manners and basic human decency you'll be okay, if not they won't take to you, the British anyway. They have a lot of islamic immigrants and when they hear your American accent they might give you crap. We are absolutely hated in their world, no matter what. It's difficult for an American to legally get a job there unless you have some highly skilled and in demand profession. If you're young enough you might get away with a working holiday visa.