British to American Universities?Watch
GCSE predictions are mainly A*'s, A's and B's.
British schools will look for AP exams and maybe SAT IIs simulating A-levels. That is what they look for with US applicants, and for some other countries that don't have A-levels and they don't trust the local exams. It is hard to get into Oxbridge from the US, but to be honest your scores probably aren't at that level anyway. It shouldn't be a problem for anywhere else.
There might be some advantage in applying in the US, as you won't be considered international, will have US grades and exams, and presumably will have gone to a good school. I am not sure the other factors involved, but if it is a decent prep school, then it might be a good idea.
You should be able to take a lot of AP exams regardless, which is what you would need for British schools. As indicated before, it would be an advantage applying to US universities, but probably not applying to British universities. It might be worth going to an excellent school, even if it will not improve your admission chances in the UK.
You can google "us prep school rankings". This will give you some idea, but there are fine schools not on the list.