How is 11th grade in London like?

I just finished my Sophomore year of high school here in the United States. I want to know how junior year will be like in London. Is there a state test? Here in the US, we have to take a SAT which measures our knowledge and decides whether or not you get into a university. Also, is it the most important year for determining if you get selected into a university? If not, what grade is? Also, if you have any other tips, it would be much appreciated. Thanks!