Best A-levels: maths, further maths, physics , if you do four: popular choices include: chemistry, comp sci, economics
Aero or mechanical?
Aero is more focused on aerospace specific applications & the lens in which you learn (for example when you do fluid mechanics, in aerospace they teach it as aerodynamics whereas in mechanical they cover greater breadth but the physics is the same .. this extends to other areas as well).
It’s really a question of preference, but your options after either degree are flexible with both.