Hey,
I am in Year 12 and currently studying Biology. There is clearly a jump between gcse and a-level biology but it will just really be an expansion on topics that you may have covered in gcse. E.g. The features of a cell is not only a nucleus, cytoplasm and cell membrane, but a whole lot more.
I believe you may know a bit about prokaryotic cells in your spec. You say biology fascinates you so I would highly reccomend you to take a level biology, because you will learn that there is so much more to biology compared to what you are learnt in GCSE, which, in fact, is fascinating. The content is more detailed, but as long as you are keen and interested in the subject, there should be no problem.
Anyway good luck with your GCSE's.