Getting into Vet School first time round is very, very hard. Most successful candidates will have between 7-10 A*s at GSCE, and no less than an A grade in 1-3 subjects. Maths, English and Science must be at A* in particular. At AS level you do need 4As, anything less and you WILL get rejected. You also need to ensure your predicted A level grades are at A*AA at the very least, but ideally A*A*A* to be on the safe side; anything less and you WILL be rejected. Work experience and extra curricular activities also are very important in the selection process. If you have relatives already working in the profession, mention this on your application and at interview, and this could give you an edge. The BMAT score, where required, must also be met.
If you do get at least AAA in good subjects at A level, and still get rejected, all is not lost. Apply the following year, and use the year gaining more work experience at Vets, farms and animal rescue centres. A few weeks volunteering for a vet in a developing country such as South Africa or India where you see wildlife veterinary medicine in practice could also make your application stand out more. If you were educated at a weak comprehensive school where there is little history in students getting into competitive courses, this will definitely count in your favour second time round.
If you get AAB or lower then the chances are you will never be offered a place at Vet School, and if you do still get an interview you will need some exceptional reasons for failing to meet the grades. The only thing that could save you in this instance is, as mentioned earlier, that you were educated in a low achieving school from a deprived area. There is the option of doing the course in another country where entrance requirements might be relaxed, but you will need to fork a lot of money upfront to afford doing the course over 5 years.