Although Imperial is no longer part of the UoL, you can still find some private 'intercollegiate halls', where you'll be with people from UCL and KLC, etc, if you're worried about people not going out from Imperial specifically. What you'll most likely find is that there will be people who go out, and people who don't, just as at any other University, and although perhaps more who don't go out than is average elsewhere, there'll still be plenty of potential party-goers.