Hi!
I'm just finishing year 13 and I did bio, maths and English lit. For my sixth form, they set us some transition work to do over the summer, once we'd had an offer holder day. For bio, this involved some building on GCSE topics we'd covered (an essential for me as I did combined science) and I also did the geography work (having previously applied to geography) and this was again bridging the GCSE-A Level gap. I personally wouldn't recommend learning loads of content over the summer but the CGP "head start to A Level Biology" was great for me as it covered the first topics of each unit and was a great way of familiarising myself with the content. I am aware that there is also a chemistry one and I assume that this is similar to the biology one.
Don't forget to enjoy this summer though!! Let me know if you have any other questions! 🙂