Melinda Menzer's very handy GVS site from Furman University - with sound files and widgets as well as diagrams and explanations of the GVS. You don't need to memorise every detail for A Level study - but an awareness of its existence can help explain orthographic changes in texts over time, especially Middle and Early Modern texts.
http://facweb.furman.edu/~mmenzer/gvs/index.htm