I agree with Leorl who, like myself, has seen medicine in both the U.K. and in U.S. The bottom line is that both systems are just so massively different that they cannot be reasonably compared. The U.S. system definitely makes you more accountable as a medical student: you (under supervision, obviously) admit from the E.R., write orders, develop a plan, round daily, order tests (must be co-signed) and are generally accountable for the details. Nothing close to this was the case when I rotated for a month in the U.K. However, the British medical students get dumped with scut a lot more than we do, and so they are definitely more proficient in starting i.v.'s, doing ABG's etc. They rock over there. I had a great month doing all of those things.
In terms of admission (I studied for fours years in a U.K. university and took classes with medical students, so I can talk confidently about this): you need good grades on either side of the pond, and you pretty much need to be impressive regardless. A British doctor from Manchester who was in the U.S. for a few years with me said he felt it was harder to gain admission stateside because you have a much harder entrance exam that is required on top of good grades. I don't really agree with that. From high school, the students that are admitted in the U.K. are impressive. The grades are high. However, for those admitted as graduates in the U.K, you can definitely get away with a lot more. I studied with people who actually failed undergraduate classes but got admitted to medical school as a graduate applicant. This is possible because the British undergraduate degree essentially awards the degree classification (sort of like a GPA, but not really) based almost entirely on the final year's work. If you had crappy grades and failures in the earlier years, it gets washed out if you get a 2:1 or a first class honours in your final year. In the U.S., you are held accountable for EVERY grade. If it's not an A or close to it, you'll have a problem being admitted to medical school, and a failure will probably jettison your application altogether.
In the end, the product is essentially the same on either side of the pond. While much of the education occurs at the postgraduate level in the U.K, more occurs at the undergraduate level in the U.S. Undergraduate and medical school training eats up more time in the U.S, and residency is shorter. The converse is the case in the U.K. In the end, both groups are around the same age when they are a consultant/attending physician. I hope that helps!