I agree that crossing London with luggage is a bit of a task, but possibly preferable to BNS. Every time I go there with Cross Country I am delayed and miss my connection, spending much more than 45 minutes there, and the air pollution at platform level can be horrendous. Furthermore most platforms have two ends, and while one can move between all platforms at one end, at the other end one can only do so by going through the barrier, which means one has to get in again. I usually end up in tears when I make the mistake of changing there.