I took those initially then dropped biology, and currently take maths chem and english lit
maths was much different to gcse
english was the same but more structured, as in you needed to know context, genre and critical perspectives. It's not too much reading but the texts are really good from my exam board at least.
chem is hard at AS but gets a lot easier at A-Level, or at least that's how my school did it. Hope that helps !
Edit: chem is also quite different in content to GCSE but also builds on it.