in response to the original question: i'm a british citizen, educated at a british public (private) school who went to an american ivy league for college. first thing i'd like to say: the two education systems emphasize completely different things. the british system emphasizes early specialization: 3/4 subjects for a-level, 1/2 subjects for undergrad. if you have a passion for one or two particular subjects and relatively little interest in studying anything else at university level, the british system is perfect for you.
the american liberal arts system is based on a completely different philosophy, and places value on a broader education with later specialization. this is perfect for people who haven't known since age 3 that they want to be a doctor, lawyer or economist, and want some time to explore various disciplines before settling on an area of concentration. i have found this opportunity invaluable: in the uk i applied for ppe-related courses, while now i will be majoring in econ and east asian studies, which is where i now realize my interests really lie. furthermore there is arguably a lot to be said for experiencing a variety of different disciplines at university, as each comes with its own different methodologies, assumptions and perspectives. to study only one subject rigorously for 3 years arguably skews one's perspective of the world to at least a certain extent: think about the vastly different approaches and emphases of economics and anthropology for instance, or physics and history, to understanding the world.
while content-wise some introductory courses taken in the first year are comparable to a-levels, it is only common sense that the level of thought, discussion and analysis required will be of a higher standard at university, seeing as the courses are taught by professors and many of the discussion sections are either led by professors themselves or grad students. this in itself is invaluable. furthermore most top colleges will let you use your a-levels or ib credits to place out of introductory classes that you've taken and start directly in higher level courses (e.g. i placed out of principles of econ into intermediate macro in my first semester). the introductory courses i've taken in subjects i didn't take at a-level, for instance literature humanities, i have enjoyed immensely and have found greatly beneficial. a lot of the quality of education you receive in america will be dependent on the college you go to; while arizona state university may well not match up to certain british universities, the likes of princeton, columbia and stanford will not let you down. once again i emphasize that the two systems are completely different, and among the top universities of both countries the only deciding factor should be which style of education suits you better. one point to remember though is that there are many more top-quality american universities than british ones, with often better name recognition and prestige (less important but nevertheless still something to take into account for global employment prospects etc).
finally, small anecdotal point about sats vs a-levels: got an a very easily in maths a-level, got 650 in my maths II sat subject test. sats are generally of a lower standard, but challenge you in different ways--particularly through time pressure.