I did all my cores in the UK (Blackburn) and prematched into a family medicine program. While Blackburn was a depressing place to live for many reasons, in retrospect I actually think my rotations in the UK were much better organized and offered more hands-on learning opportunities for students than most of the rotations I have done in the US. I also don't really consider the fact that I was able to skip out on the opportunity to stand around for hours doing mindless scut a "disadvantage"

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As for how it was received during interviews, I would say it was 95% positive. Most programs wanted to know what I learned from my experience and how it has shaped my opinions about the current debate around health care. There were also questions about my interactions with people of different cultures, backgrounds, and beliefs. If nothing else, the experience gave me plenty of unique things to talk about. There were some experiences - namely my week in the TB clinic with the guy who authored most of the WHO guidelines for TB, and the daily opportunities for procedures (joint injections, phlebotomy, suturing, immunizations, deliveries, etc...) that I made sure to highlight in my interview.
This being said, you want to make sure that you do enough rotations in the field you want to match into when you get back to the US. All the programs I interviewed at wanted to see strong letters of recommendation from US doctors. For fields like surgery, I think it would be a good idea to keep a very detailed case log of the operations you assisted in while in the UK (being first-assist for a whipples procedure as a medical student probably won't happen in the US, but it is a very real possibility in England). Since most US medical students I've talked to only have to do a 1 month surgical core anyway, I don't see how 12 weeks of awesome case logs in the UK could hurt you -- as long as you made sure to concentrate on surgical electives and good US LORs back home.
That's just my two cents.