The most common route into medicine is via A Levels, with grades AAA or better in three of Chemistry/Physics/Biology/Maths. If you aren't taking the right A - Levels, study an extra year to get the right ones and you should be OK.Some Universities are flexible, accepting A*AB, or taking family background, circumstances and extra curricular activities into account. A few years ago a student got a C in A - Level Maths because her school failed to teach her a complete module, but she was nevertheless accepted into Kings College. These more flexible Universities tend not to be the top grade medical schools and making concentrate on training for GP practice.
If you don't get the right grades to get onto a medical course after A level, you might apply to a private or overseas University or take an allied degree like biomedical science and take medicine as a postgraduate course. The last route may add an extra year to your studies,