Originally Posted by ninety_nine
Unless you plan to stay on in the US, I don't know why you'd be prepared to go to an American university - the fees alone are enormous!
Some of the top US universities recently (i.e. as of 4 years ago) began to run needs-blind admissions processes which apply to international as well as domestic students. Prime examples are Harvard, Yale and Princeton. Schoalrships and bursary support can also be won from universities and from the
Fulbright Commission
Originally Posted by ninety_nine
I'd imagine it'd only be worth it if you got into one of the top american universities i.e the Ivy Leauge, M.I.T etc. Even then you could just apply to Oxbridge/LSE/UCL/Imp etc. saying you a lot of time and money, without losing any prestige or graduate prospects.
As for prestige/graduate prospects, it may seem like a fine line, but the Time Higher Education supplement while not exact, has the following big picture outlook for its World University Rankings 2008
Top 10: US has 6, UK has 4
Top 25: US 14, UK 6
Top 100: US 36, UK 17
It's also worth noting that the trend is in favour of US institutions. Those UK universities in the top 200 are falling, including Oxford and Cambridge, which now lie below Harvard and Yale.
Back to the OP, aim high. Many US universities have little clue as to the difficulty of A Levels, and so are looking for good results. It's worth scouting out the opportunities at some of the mid level US colleges, particularly the small liberal arts colleges. They provide excellent facilities and faculties, and can be a great opportunity to challenge yourself, broaden your horizons and forge lasting friendships.
But until you take the SAT you'll be hard pushed to know exactly where you stand relative to US applicants.