To give you an idea, if you take the top 35-40 National Universities and the top 20 National Colleges, here are those ranked 29-40 according to USNWR
Georgia Tech, Tufts, UNC CH, URochester, UCSB, UF, UCI, Boston College, UCSD, UCD, Boston University, Case Western Reserve, William&Mary, Brandeis, Northeastern, Tulane and the top 15-20 national colleges (which are roughly as hard to get into as UC’s, URochester, or UNC CH) Grinnell, Hamilton, Vassar, Colgate, Davidson, Wesleyan, Bates. All are considered elite and extremely selective, even more so for transfers than for freshmen since few freshmen transfer out.
For those, as a freshman, your odds as an international are probably half of the regular acceptance rate. So, as a transfer, perhaps half of your odds as a freshman?
So, about 5% for Bates, Hamilton, Tulane. A bit more at Northeastern, W&M, Brandeis, BU. 10% at most.
I’m assuming you scored in the top 10% for your country in secondary school examinations (which I infer from your being admitted to a top British university).