Congrats on getting an offer! I'm a first year biochemistry student. I cant offer much about the social life since I personally don't do a lot of partying/drinking, but I do know lots of people who go out clubbing multiple times a week. The night life in london is pretty good, and imperial having no social life is definitely a myth. However, the workload is intense and you can tell that most people come here to get a degree rather than for the university experience, ygm. If you want to get a first then I'd say you probably have to sacrifice some of the social life, but people aiming for a 2:1 can enjoy a good social life.
Living in London can be overwhelming, but I personally really love it. There is always stuff to do and places to go, so you will never get bored. It can be expensive, but with sensible lifestyle choices and the generous imperial bursary it is definitely manageable.