OK, now that you haven’t perhaps got the grades you had the potential to achieve; you should work on primarily your A-levels of course, but secondarily, work on your UCAS... Make yourself stand out.
Becoming a vet is as you understand very competitive, and at top Uni’s such as Cambridge, you pretty much DO need a set of A*’s, that’s the realism of the situation. That is not to say that going to any other Uni is bad? You must do research into what Uni’s look deeply into GCSE’s… there are, as I’ve looked into in medicine, some university’s that do not look at GCSE’s and are more interested in a non-academic orientated person (that is not to say they will accept you with bad A-levels of course).
You should definitely not feel down about your situation, as in fact you are fields ahead of many others, with you work experience, and if not that in large quantity, you understanding for the components needed to become a Vet.