Kinda repeating what Cucumber said, but:
All vet schools require chemistry and biology.
Only Cambridge and I think one other vet school which I can't remember off the top of my head ask for maths or physics in addition.
Provided that you meet these requirements, no combination of subjects is preferred to any other. Doing bio, chem, phys & maths would not put you in a stronger position to anyone doing bio, chem, maths/phys and another subject.
As someone who takes three sciences and maths (in year 12 currently), I wouldn't really recommend it and if I could choose again I would have perhaps done geography or philosophy in place of maths. The benefits of my course are that there are no essays (so easier exams, in my opinion), plus every question that can be asked is already in past papers so when it comes to it revision will be fairly easy. I also get relatively little homework compared to my friends who do history, philosophy, psychology, economics etc. However, I also feel that my writing skills may have suffered a bit, and wish I had kept at least one essay subject in order to remain a well-rounded person. Maths is a lot more difficult than I anticipated, and I will definitely be dropping it and the end of this year.
As no subject combinations within the requirements offer an advantage over others, I'd recommend doing biology, chemistry and two subjects that you find easiest and think you will enjoy the most
If you have any more queries about A-Level options or any of the subjects I do feel free to drop me a PM