You honestly don’t need to go ahead and do any A-Level work in advance, I didn’t and you won’t necessarily be at a disadvantage if you don’t. This summer is the last summer of freedom you’ll have, as next year you’re going to be busy doing your UKCAT and writing your personal statement (and the list goes on!). so just enjoy it and completely relax!
However…if you are still determined to do something, I’d say to make sure you know the GCSE content for your sciences really well, particularly the triple science units. You’ll be so glad you’ve done this when it comes to starting, especially for A-Level Chemistry as about half of year 1 is covered in the triple science units. Make sure you’re properly up to scratch with it all so when it comes to building up on presumed GCSE knowledge you’re not struggling.
There’s also a series of books you can get from CGP called “Head Start to A-level“, which is supposed to bridge the gap and give you a boost on to starting A-Level.