UK undergrad, USA postgrad is the standard move when faced with a decision like this
US universities make you major/minor in various things which means you come out worse at your core subject than peers at UK unis who've just done that one thing and nothing else for 3 years. So if you want to just do biology and that's what you're interested in, I'd recommend Oxford. If you want more breadth but less depth in your education, Harvard is better, but I'm guessing that's not the case given you didn't apply to Cambridge for NatSci.