If you take Foundation Maths and Sciences you're unlikely to have the grades to be allowed to study A level Sciences and that will prevent you from becoming a doctor. If you're entered for Foundation (as your school thinks this is your best chance of getting the best grades) it also suggests that you lack the work ethic and/or the ability to cope with A level Sciences or a medicine degree. There are exceptions to this - e.g. you have newly arrived in this country and your grasp of English is poor - but it applies to most cases.