You have to be in the country at least 5 years to be a citizen.
Also, if you are looking to get into Harvard, Yale, or Princeton, it is easier if not international, but it depends if they want to take you. Having great academics doesn't get you in. Together they take about 5000 students a year in a country 5x the population of the UK. At most 40% of those admitted don't have "hooks". Hooks are like being a recruited athlete, parents or grandparents or whatever are big donors, high class or political connections, being black, Hispanic, or native American, or being from an underrepresented state like North Dakota or Arkansas or something.
If you have top academics, you will get into excellent US schools though, but probably not the very top. You can also simulate A-levels with AP exams and apply to Oxbridge, LSE, etc.