What is the perception of foreign Universities (Non UK) in the US for prospective academic positions (Postdoc/Tenure)?
I don't think there's as much of a perception problem with foreign universities as you might think. I'd say the majority of postdoctoral associates are international, and most of them have not gone to universities as well known as Oxford and Cambridge. More likely than not, forging personal connections with the advisor you want to work for as a postdoc will matter much more than the university. 

That said, individual professors may have a bias toward hiring people from "known" schools, because the risk factor associated with hiring them is presumably much lower than someone from a school they've never heard of. Such a bias is of course understandable, but it also means that they could lose out on some potentially promising candidates as a result.