In England, grade XI (GCSE "year 11") is mandatory, but you finish it at 16 years old and can leave school. Grades XI and below are 1 to 2 years behind North American schools in terms of content, but that is because GCSE's have been decreasing from what they once were (OLevels).
Then you can do what are called "A-Levels" for grades XII and XII. For students who take them, these make up the short-fall in GCSE's if you take STEM subjects (Science, Technology, English, Mathematics). They cover the "missing" 2 years content, and then things that may be taught in the first year of university in North America. I've only looked at Canada, but a B or above in a Physics A-Level is equivalent to 1 university credit in Physics. This is usually the case for each STEM A-Level, but some are worth a lower value of 0.5 credits in their subject.
I haven't been to university yet (still finishing A-Level exams), but I've noticed that it's more the university you go to, rather than the course you take, that affects what you learn. If you go to Oxford, you get one of the best educations in the world. If you go to some other places, you don't.
Most of my teachers often tell me that the education system here is much easier than it used to be, and that what we do at A-Levels used to be done in OLevels (old GCSE), and their A-Levels had the same content taught in universities today. I wouldn't believe it if it wasn't for news showing 8 year olds getting A's in the papers I'm taking this summer
As for the pressure of doing the subjects, I found GCSE's came to me fairly easy and I didn't have to work hard to get good grades, but others do not have the same ease.
A-Levels are varied. Some people do IB's which mean they need to take 6 or so subjects for A-Level, while others only need to do 2 to 4 A-Levels. It also depends on subject, as science (physics, biology, or chemistry) and mathematics are regarded as the most difficult subjects to take. I think that every A-Level student will feel more pressure than when they did GCSE's however.