It completely depends on the metric you use to measure them. I don't think it's true that all medical schools are equivalent. The simple fact that they are all GMC-certified just means that they meet a (demanding) minimum standard. For example, students graduating from certain medical schools consistently do better on Royal College membership exams. That would be one reasonably objective measure. Not to say that it is possible to quantify exactly what makes a good doctor, but that doesn't mean all schools are the same.