Not sure about the AP stuff, as it's been over a decade since I've taken them, but maybe I can help with the other questions.
I went to school in the US, but studied abroad at Goldsmiths for a semester. I then came back to the UK in 2007 to do my Master's at Goldsmiths. I think that starting university is always a bit of a culture shock, but starting university in a different country will present even more challenges. You will have to learn about a new country while doing your studies, and it requires a certain fortitude to deal with making friends, having a social life, dealing with a strong drinking culture and a host of other differences right out of high school if you are unused to it. I'd already learned the system through study abroad at age 21 and it was still a bit of a shock doing my MA a couple of years later!
An important thing to keep in mind is that not only do you have to pay tuition fees, you have to pay for possible lab costs (sorry, as a liberal arts student, I have no idea what's involved with lab work) housing, living essentials, laundry and food. Universities in the UK are not catered like they are in the US, you will have to buy and prepare your own food, as well as buy a reasonable amount of crockery and cookware. Also, keep in mind the costs of student life (pubs, clubs, etc.) As a foreign student, you will not be able to work very many hours without breaking the rules of your visa. Since you have picked unis away from London, you will have the benefit of lower costs of living, for example- my MA cost me a little over $40,000 for one year, once I factored in housing, food, leisure and transportation. Still cheaper than school in the US, but not cheap in any sense of the word, and now I have to deal with substantial loans.
You will have to take out loans through the US (and you will have to pay them back, regardless where you end up living in the world). Your uni may offer emergency loans, but those have to be paid back before you graduate. British students taking out loans to attend British universities do not have to pay their loans if they make under a certain income (£21,000 annually), but they still have to pay back their loans eventually. These loan rules do not apply to foreign students at all. There may be international scholarships that you can look into, the British Council may have more information.
Student visas have changed since I came here, and it's a lot more difficult to get in the country and stay in the country once you graduate. So keep in mind that if you learn a subject that differs to what's taught Stateside and you have to return to the US, it may be that it's not worth the expense and 3 years study in a foreign country for a set of non-transferable skills. Also, if you do apply to any UK universities, make sure to apply for your visa at least 5 months in advance to avoid missing out on actually starting your course. You will also have to pay £298 for the application (about $458) and pay for biometric information as well, so there will be a fair bit of money being spent, to put it mildly.
I hope this helps!