Hello,
I studied at King's College for a year, and I took all science classes. Personally, I feel that if you can take a year abroad, then do it. It was the best year of my life, you meet a lot of great people, and you get to do a lot of travelling. But, if you only want to take a semester, then it really depends on your situation. Going fall semester means that you will have less time there and some bad weather and that you will have exams right at the end of the classes (exams there are worth 70% at least of your grade). But, on the plus side, you do get a reading week for travel (well, to read, but who really does that?!), you won't have to worry about MCATs, and they tend to be less hardcore on their exams if they are just for the foreigners leaving. In spring quarter you get lots more time there, a three to four week vacation, and exams after that vacation. If you are taking the April MCAT, you can take it in England...it is a bit more expensive, but they give you champagne when you are done. Keep in mind though that I think it falls in the middle or end of your spring break.
So, really, the point is, if you are taking the April MCAT, it can be done abroad. But, if you are only going to study abroad for opportunities to travel, then the MCAT pretty much kills your travel time. Also, when the exams are only matters if your grades transfer back. If you are just doing classes P/F, then it really is not that big of an issue.
Well, I don't know if any of this information is very clear, but feel free to e-mail me with any questions you may have or if I need to clarify something that I wrote!
Good luck to you!
PS Take a year!! It can be done!!