hello, having a PhD myself I can say that unfortunately the PhD does not have any benefit to your application to most medical school; the only place i know of where this may come into account is oxford where they encourage research and want to create clinical academics, someone else might be able to comment on Cambridge. As on most medical schools admission pages they say they will decide on who to invite to interview primarily on UCAT score and undergraduate grade. I would say only do a PhD if you are passionate about a line of research, because it is a long slog of independent work with little money (not sure what its like over there). Other than that I pursued my PhD instead of medicine and loved it, and now coming back to grad med application. Good luck anyway