Let's get together a list of web resources for studying biology:
CollegeOpenTextBooks.org- Incredible list of freely downloadable biology books at a variety of levels. Many are more targeted at US college level, but this is often also useful for UK A-levels and GCSEs. If you get stuck with your official school book or your teachers notes, you'll find a book that explains things at the right level for you.
Howard Hughes Medical Institute - A US medical research institute with fantastic animations and short films illustrating important medical concepts. Definitely more slanted towards university level, but lots of stuff still great for A-levels. It might take some looking but its worthwhile - as the animations are very powerful for understanding more complex concepts.
University of Utah Genetic Science Learning Center - Another site with lots of very powerful animations and very well designed.
Khan Academy Biology - Khan Academy is mainly focused on maths, but also has quite a few videos on biology at a secondary school level. The videos consist of guru Sal Khan explaining concepts in a very accessible but still serious way on his tablet and you'll only see his drawing on the tablet.
MIT lecture videos - MIT has an amazing collection of videos of university lectures. Many are at higher university level, but some are also quite foundational and so are certainly good for deepening your understanding for A-levels
ToKToL Biology - Adaptive self-study platform covering evolution, cell biology and genetics for biology A-levels; you select the topic and study by answering questions that are repeated if you get things wrong; questions get harder as you get better
NIGMS - the US National Institute of General Medical Sciences provides a great list of general biology texts, for example on cell biology, genetics, very well illustrated and written and highly accessible.