You'll need A levels or equivalent in Maths and Physics, usually A*AA or AAA overall though there are unis which require less and a couple which require more.
Yes you can do Mechanical first then specialise in Aero, either via the university offering it as a specialisation, them having the Mech and aero students on a common core, or doing an MSc. My word of warning regarding this would be that you could miss out on a lot of content which you would've done if you had done just Aero. At many unis you can't transfer between the two courses because the differences are so large.