I'll try to help you out here, but I must warn you that my only experience with US schools is as a student 20+years ago.
The main difference in my opinion is the age thing. Children usually start reception(kindergarten) when they are 'rising 5'. This means they are in the school year September-August that they are going to TURN five. In the US, it's after they've turned 5. My children are 13, 12, and 5. They are in years 1, 7, and 9. Only the seventh grader would be in the same grade in the states(Sept. birthday). The other two would be a year behind. It is unheard of the hold a child back. Children struggling in school do not get held back either.
There is a huge emphasis on testing. Children are tested in year 2 (SATS) year 6 (SATS) year 9 (SATS). League tables of all the schools in the country are then made up judging on how well the schools did on the year 6 and year 9 SATS. So these are VERY important, and alot of time (too much IMO) is spent revising (studying) for them. In year 10, students start studying subjects they would like to study for GCSEs (more tests). Mine haven't reached this stage yet, so someone else (Peedal?) would probably know more about it. How well they do on these tests determines what they're going to do. They can legally leave school at 16. A levels are the tests they take to get into university. They study for these for two years-taking exams called AS levels the first year and A levels the second years.
Most schools wear uniforms. It's in my opinion a more formal system in some ways, but things like recess are far less structured. Mainstreaming seems to be years behind the states.
Hope some of this helps. Feel free to ask any questions. We try our best to answer.
Good luck