Yo, ICL student in Math here, class of 2020. I was in the same building than the computing students. There is definitely a lot to do outside of the uni and many people have a well-balanced social life as long as you're engaged in societies (e..g, sports "lads"...) and going out (although this might be refrained due to COVID for the next academic year). For so', once you're on campus most people are there to work (it's Imperial after all, the standards are really high), so don't expect much on campus, but trust me there have been memorable sports nights and night outs.
To me, personally seen your offers, kinda no brainer to firm Imperial, and then if you like put Leeds as second choice. Would you not regret turning Imperial down? Would have been a much tougher decision if Imperial vs Oxford tho.