I don't know of anyone who went to uni at 16... it's generally 18, after A Levels or whatever.
I know the system here is different to what it is in the US, but Audrey T seems to have offered some good info, go to that website and whatnot. I know we do allow foreign students, but I don't really have much of an idea about the specific conditions (and definitely no clue as to the deal with certain universities)
Anyway, erm, totally off topic, but... I know Brunel University. I used to sit in the surrounding fields and get absolutely wasted with a bunch of people :-) Once there, we came across one of the safest security guards ever. He gave me a fag and told me to stop listening to weedy music >_< Some of A Clockwork Orange was shot there. How very interesting.
You'll need to look this up on the school's website. I'm sure they'll have a section for students from other countries. It's probably different depending on the place you go. Unless someone else has done this, I don't think anyone on here will know the answer because US vs English schools are two different systems.
Definitely take the SATs (and ACTs if you have the chance). Even if you're thinking of going to a college out of the country, you should still take them. Most colleges don't considered applications from students without them because it's the one standard form of testing a students ability that they have.
As for accepting credits from other colleges, it's really something you have to discuss with each college you're thinking of attending, since each college's rules differ. Some colleges only accept grades of C+ or better, some colleges only accept credit from certain other colleges, it's all very specific. So maybe you can get in touch with someone from the college you're considering going to.
Edit: This site seems to have good info on international students attending school in the UK: http://www.ucas.com/students/wheretostart/nonukstudents/
Carl B. went there if I recall :D
I hope you'll find where you really want to study ! :)
xoxo
~ Violette.