There were two ex-US students when i was at medical school. They were treated as any other none-EU international student.
Grades are difficult but universities that accept international students for medicine will want to see your GPA and SAT scores. If you contact the university admissions department they will tell you what they require. Applications are online and the UKCAT has centers in the US, but not many.
It's worth remembering that as it stands today, you must complete medical school AND foundation year 1 (similar to being an intern but better pay) to qualify for full GMC registration. If you wish to return to the US to practice then you still have to sit the USMLE and start again from the very beginning.