How are British PhDs looked upon in the United States?
I am an American who earned a Ph.D. at Cambridge University. A degree from Cambridge or Oxford is well-respected in the US - I was warned to avoid other UK universities as they are not as well known in the States.  The one caveat is that all of your network for post-docs and jobs will be in the UK, not in the US. It was an amazing experience though so I would not want to discourage you!