can someone call clearing on my behalf or do I have to call personally?
You should call yourself because you still need to attend an MMI afterwards if you're eligible for a place at St George's so I think there still might be competition for places. Besides someone calling on your behalf would only weaken your case
can someone call clearing on my behalf or do I have to call personally?
You don't call clearing, you call the universities you want to go to.
You're going to need to call yourself, pretty much. You're calling to ask for a place, it won't look great if someone else calls, especially if it is for medicine.