<p>I would say it is a combination of both. An IVY or another top 30 institution such as stanford, UCLA, Emory, John Hopkins would be well known universities in Europe and would impress the admissions office. It would be hard to transfer from a USA university to a British one because of the two very different systems. You are better of just starting an undergraduate at a British one or doing a JYA in your third year. I know Oxbridge is hard to get into but as an American you might have an advantage because there is not a huge amount of undergraduate students at those unis from the USA.</p>