I agree with Silverstar.
Also, to clarify, just about all the vet schools require 4-10 weeks or more (depending on the school) for work experience. So getting some before applying is not just a good idea, it is pretty much a MUST. Many students shoot for 15 weeks+, in order to be competitive for Liverpool, as they are the most demanding on work experience.
Biology will be a must, as will chemistry, probably minimum A. Look at the different schools for specific requirements.This is unless you do a Gateway program, which is also competitive and has strict requirements-again, do your research.
If you apply as a graduate student-do a degree first-you might have a better chance with those A levels, assuming you do well in your first degree, but there is virtually no funding available. So you would be looking at paying 9,000 to 28,000+ per year for 4 to 5 years out of your own pocket.
I also agree that work experience is vital to helping you decide if this career is for you. Therefore again, I strongly recommend you work at getting some. See the work experience "stickied" thread up top for guidelines but in general you want small animal vet, large animal vet, cattle (especially dairy), equine, kennels (cat and dog if possible), lambing is strongly recommended, and whatever else you can get.
Good luck.