In London - aside from office workers, uniformed roles, etc - people wear whatever they like. It's absolutely the fullest spectrum you could imagine. Students are especially experimental.

So there is no right or wrong, just be your authentic self and dress how it pleases you and reflects your personality. T-shirt and joggers absolutely won't seem out of place. So rather than try to predict from a distance how you might wish to dress to fit in, come over with what you've got and just buy new stuff as and when you know what you actually want.