As other people have suggested, I would advise that you go and look at the two campuses (ideally in term time, which starts on the 24th of September at UCL, and early October at Imperial). Go to the student unions, chat to people, get a feel for the places. Apply to the one that you feel would suit you the best, not to the one that you think is earsier to get in to.
If you want to see how active we are socially, come to UCL's Huntley Street bar any evening of the week beginning the 24th, or this Monday or Friday to the Royal Free bar in the Royal Free Hospital for the 3rd year parties.