My dad was a doctor and he used to say the smart guys were the vets. Dad only needed to know how one kind of body worked and most of his patients could describe their symptoms or at least say where it hurt, but a vet needs to know about everything from budgies to horses and -a part from Dr Doolittle, he can't ask his patients anything!
You need to know that As and Bs in GCSEs won't get you anywhere near university for a medical degree, don't you?
They will insist on a fistful of Highers or A Levels, normally all at A or B in a range of subjects that wil include English, Maths, Biology, Chemistry etc, just to even qualify for an interview for a place, then you have to pass the interview where your commitment to the course will be tested - they won't start training anyone who will likely drop out part-way through. There are so many candidates that they can afford to be choosy.