The only A Level I revised for was Business Studies.
I didn't do it at GCSE level, so it was all new to me.
After AS year I was on a a fairly low D in it, but I really wanted a C at least.
In the A Level year I revised for the exam in January and 2 exams in June and I managed to nail them, without looking at my certificates I'm sure I got a B, B, C which I come out with a high C overall.
Passed my other A Levels without any revision, but for subjects your "worried" about, revision is well worth it as I am sure you have been told by many people