Generally A-level Maths and Physics are required for admission. Some universities also require a third STEM subject, which is commonly A-level Chemistry or A-level Further Maths, although these subjects are rarely if ever required formally.
A-level Further Maths is desirable to take if your school offers it, even outside of admissions considerations, because you will learn that material on an aerospace (or any) engineering degree sooner or later. It's easier to take it at A-level when you have much more opportunity to get individual attention from your teacher if you are having trouble with a topic, and where you will spend longer learning each topic as well (spending weeks on a given topic compared to just a lecture or two on it in uni).