check with individual universities about changing A level subject, but you'd need at least AAA for ANY med school, and why are you so fussed about russell group? As far as medicine is concerned, it doesn't really matter where you study, since you're all on equal standing at the point of graduation. Choose the university you like (location etc), the one you're more likely to get into or the one whose course structure you like (like for anatomy you can be taught by dissection or prosections etc)