If it's an American school in London, grading with American grades, then you don't have the added problem of converting grades.
However, I wouldn't necessarily recommend taking high-stakes courses abroad because like others said, you want to have time to travel and do other things.
What I would recommend is to still study abroad but see if you can arrange your schedule so you're taking less stressful/less important classes while you study abroad, like knock some electives out. Even science electives compared to the main subject courses (physics, orgo, chem, bio) would probably be better. At my college both physics I and II were offered both semesters, and I think the same for orgo. So if that's an option I would just come back and do physics II and orgo II the next fall. Or, study abroad in the fall of your junior year? Then you can come back and use the spring to study for your MCAT. Maybe your study abroad office will have contact information for someone in your major who studied abroad who could help you with this.