Yeah, I'd say that it is manageable, although I'm only doing AS, it's pretty easy to self-teach. I've self-taught most of the course, because I find it difficult to learn from teachers. It's a memory test - just memorise the content (with understanding of course) and do past papers, and you can't go too wrong. There isn't anything really difficult to grasp conceptually, it's just about being able to remember it all. Chemistry would be a much harder A-level to self-teach in my opinion!
Are you going to be studying anything else at the same time? If you're doing just biology, then yeah, definitely - you can do it! You might have to try and find an exam board that doesn't involve coursework though? Not sure how that would work... Also, I'm on Edexcel, and my teachers are required to confirm that we've carried out certain practicals, so again, I'm not sure how that would work...