I agree with the other vet student that you don't really 'need' to be doing anything over summer - for the most part, enjoy it! You are going straight into a very intense degree so some R&R is needed. First year is intense, but the first 6 weeks are spent doing some basics of like biochem (mostly the bio aspect) so you have time to get to grips with things then. If you really want something to do though I would do the husbandry and welfare subjects as the other poster said, but if you really do want something more then doing a tiny bit of anatomy and physiology wouldn't hurt as that is the most 'intense' topic in first year - it is just never ending lmao and involves tonnes of memorising. A book that I think is an ok bridge between A Levels and first year and doesn't break the bank (I would also look to get it used, e.g. market place, ebay etc) is Aspinall's 'Introduction to Animal and Veterinary Anatomy and Physiology' - I do think it is a little too simple once you are properly into the course, but it covers the basics which you won't know from A Levels so could also be used for the first semester or so fine. Otherwise I wouldn't recommend buying vet books until you are on the course - the library has all of them for free and you can access most of them online, so you either wouldn't need to buy them to use them (I feel the majority of vet students don't heavily rely on books as the lectures have most if not all of the info you need) or you can at least 'try before you buy' as there is so many different books it is best to work out which you prefer most before forking out (and again, if you do I recommend buying used). But then I also have a hefty collection just because I like to see them on my shelf (mostly gathering dust though apart from the few I occasionally do use) but obviously not necessary.
I will say that only revising a few days before an exam for vet school probably isn't the best idea. I also didn't really do that much for GCSE and A Levels but you basically cover the content of your 3 A Levels in a single semester (or at least feels like it lmao), and because Liverpool has a spiral curriculum rather than modular, you get tested on literally anything from the entire year in every exam so there is just way too much content to go over in a few days as you can't target your revision. I'm awful at revising during term time but I'd say you want to start revision a few weeks before the exams at least.