Hello Lorien!
In England, exams are slightly different. Boys either change school at 11/13, girls can do either, but it is far more common for them to change at 11. SATS are just exams. These are not really entrance exams, but more of a serious end of year 2, 6, and 9 exam. They are taken into consideration.
More importantly, GCSEs are taken in year 11 (16 year olds) and are serious exams, taken in a choice of about 12 subjects. Normally, you would do maths, sciences, French, classical Greek, Latin, History etc. etc. although the choice is huge (programming, PE, games, photography, cooking, food tech etc. etc. My school would never allow me to do any of these subjects) Although A Levels are more important for university, when you apply to university, you haven't got your A level results back, and it all falls back onto GCSEs. They are becoming easier and easier, with less and less material, to such an extent that you still struggle to get into Uni with straight A*s. Vast numbers of students from accross the country now get straight A*s. GCSEs are important.
AS Levels are taken the next year. They are taken in about 5 subjects, what you will specialise in. For me, Double maths, and all three sciences. Again, these are important.
A Levels, are taken the next year again. You usually drop a subject, three or four. For me, double maths, chemistry, and physics. These results don't come back until your university place has been provisionally accepted, falling back on GCSEs, AS levels, interviews, and extra exams.
You can then take a gap year if you wish. I wouldn't. It is just a wasted, unproductive year, and makes you worse in maths/sciences, and better in things like English, where more maturity is better. Then how do you support yourself, etc. If you wish to go to a posh university such as Oxford/Cambridge (shortened to Oxbridge) then you need additional exams and practice.
University starts at a high level. There is no revision. The plough/plow on with very advances topics, unlike American universities. If you haven't got the A Levels, university is either out of the question, or next to out of the question.
You then do your degree, with master's or doctorate as well, possibly.
My father is a visiting professor at a university

Some people don't like him very much either. I cannot go into details, but he is a doctor, and went back to university to do another degree. His thesis proved that some ethnic backgrounds are more susceptable to frostbite and cold injuries. Now with that proven, can companies use those ethnic people in cold places. What if they get frostbite, and call a layer, and show this thesis as mistreatment when they knew the risks. He wasn't very sensible to write it, actually.
Anyway!