I am American born & raised so I understand your confusion.
To try to answer questions, and to go back to basics:
Year 11 in England is the school year when they turn 16. It is quite unusual for kids to go up a year or be held back, so assume it's set in stone what year she'll be in. Will she turn 16 next school year (1 Sept 2013-31 Aug 2014)?
GCSEs are typically (for now, subject to future change) awarded as part of 2-3 course, including a series of tests and some coursework (projects/homework). But you're talking about bring her in for just half of that time. If she's a straight-A student the odds are high that she could get decent GCSEs even if she just did half the coursework, but the results may well not be what you want to get her into American Universities (how to do that from Britain is a whole 'nother thread).
If you speak to individual schools they may suggest different things, including purely exam-based GCSE awards (IGCSEs). For these she wouldn't need to do 2 years of course work, she would just study her butt off to pass 2x2 hour exams held over a 10 day period, which could be administered at almost any time. You would pay exam fees for the IGCSEs, I think typically about £100/exam. The IGCSEs are not all that hard, my friend's HE 12yo has passed them and he's only moderately bright.
"College" in England, also called "6th form college", was traditionally only for university prep, and targetted at kids age 16-19. At College they specialise in just 3-4 subjects; on its own, rather unsuitable prep for most American universities, I imagine.
I wouldn't know if any London or Surrey state schools offer IB; your googling is as good as mine for that.
I am wondering if she could attend yr11 here but study for high school diploma equivalent (GCE?) in your home state, in meantime. If there's someway she could swing that and then the pressure is off about GCSE results.
I hope that clarifies a few things.