I think it's good to have aims at this age. It gives you motivation and something to work towards. And if you change your mind, it's not drastic, it's not like you've cut off all other options.
I suggest that you research all the Ivy Leagues, get thoroughly familiar with the American application process. The more 'inside information' you have, the more confident you'll be. Do it now, while you have the time, it'll save a lot of stress and bother later when the time comes round to actually apply.
Also check out scholarships provided by the UK and see if you are ellegible for those. Unless you can afford to pay international tuition fees?
However, you will be competing with other applicants who all have top grades and a lot of EC's. There's really nothing special about it in the eyes of the admissions tutors.