anything can be self-taught if u work hard enough
the specification/syllabus, at least for edexcel, requires knowledge of the entire a level maths spec/syllabus, so u might want to try a2 maths first to see if u want to continue with further maths or not
as maths is supposed to be a nice little "bridge" from gcse to a level, so isn't too difficult
a2 maths can easily mastered if u put in enough practice, but it's a level up from as maths