It is quite common for students within the US to transfer between colleges after their first or second year of undergraduate (I know a guy who went from University of Texas to Georgetown in Washington). Of course, as you have presumably read, it is very difficult for them to transfer from a non-Ivy League college into one, though it is posssible (and not all Ivies are equal - it's probably easier to transfer into Cornell than Harvard, for example).
As I understand it, this is possible precisely because all US colleges operate a credit system (modules are weighted and each afforded a certain number of credits, and one graduates from college after obtaining a particular number of credits). As a universally recognised standard, these credits are then transferable between colleges, making the transition easier.
But British universities conduct their affairs on an entirely different basis and we have no no equivalent credit system here in the UK (not one that tranfers between colleges anyway). So I believe it would be difficult, if not impossible, for you to apply as a "transfer student" - certainly not like any other US educated person would, and you may just be treated like any other first time applicant (EDIT: ok that's not quite true, you'd still have one year of UCL education behind you, and I guess that would distinguish you from other applicants, I'm just saying it's not clear how they'd treat that). The best thing to do is to contact the undergraduate admissions office at a US college and explain, very precisely the exact nature of your situation and educational background, and see what they say.
Are you aware that as an international student, the opportunities for financial assistance from a US college are very limited, practically non-existent in some cases, and the costs of studying are vastly more than ours (like $30k per year in fees alone at Ivies)?
Also - why do you think you need to go from UCL to the US before you've even started your degree? Aren't you pre-judging the issue a bit? And your habit of putting entire sentences of your post in bold is quite annoying.