You'll find most people have A*s and As at GCSE with a few Bs, then at least AAA at A level generally. if you only had AAB you could only apply for vet med at Nottingham which reduces your chances a lot! 400 interviews at notts this year out of 2000 applicants, for about 100 places.
Work experience is quite hard work and time-consuming, and is a big part of a vet application, so its worth thinking about if your brother hasn't yet. E.g. Liverpool ask for 10 weeks minimum (4 at vets, 6 other eg farms, stables, kennels).
Although other universities do not set such high limits, many applying will be also be applying to Liverpool and so will have a lot of experience, upping the competition!