Assuming the train has 8/10 coaches, it is usually less crowded at the back, so easier to find somewhere to put your luggage. If for some reason it is only a five coacher, that is not so good. If you have luggage, I would get a taxi from Waterloo, just to avoid grappling with escalators, and a very long walk at KX/St. P. If you don’t have much luggage, the 59 bus goes from just outside Waterloo to just outside St. P.