According to the Royal Veterinary College, the average offer for a Veterinary Medicine course at UK universities is AAA - HOWEVER - this will differ depending on the university you apply to.
For a medicine degree you will also need to sit the UCAT - this is the University Clinical Aptitude Test. Each year they have different deciles that some universities use as cut off points for considering applicants.
UCAS: https://www.ucas.com/explore/subjects/veterinary-science
MUST HAVE STUDIED: Chemistry/Biology
USEFUL SUBJECTS TO HAVE STUDIED: Maths/Physics/Further Maths
University Subject Guide: https://www.thecompleteuniversityguide.co.uk/subject-guide/veterinary-medicine
Work Experience Support: https://www.rvc.ac.uk/study/undergraduate/bachelor-of-veterinary-medicine#tab-work-experience-support-programme