I went to Imperial as a post-graduate and all I can say is that I am sooooo glad I didn't go as an undergraduate.
It is a great university and suits some people but it would NOT have suited me at all. I went to a small university, in a town where you could walk most places, I did lots of 'clubs and societies' stuff and socialised at house parties. I was not into clubbing and preferred pubs and the occassional cheesy student 'bop' at the union.
Imperial only has science students and medics. I did some philisophy modules in with my maths and physics degree and most of my friends were arts students - philisophy, english, anthropology... As a girl doing physics I found the nerdy male atmosphere a bit much at times and was glad I had friends from halls from other subjects. The humanities subjects on offer at Imperial are very much 'enrichment' for science students.. though the free language courses are good (I did Spanish).
Many Imperial students when I was there for post-grad were from countries with extremely strong work ethics; china, singapore etc. They worked VERY hard, spoke very little (to me anyway) and did not really soicalise much. I am very into clubs and committees and voluntary work (which led to my current profession) whereas the Imperial students were very single-minded in the pursuit of the best possible grades in their subject.
Also, I liked London well enough but it's tough as a student with no cash at all, many Imperial courses make it almost impossible to work due to lab hours etc. I managed with some late-night waitressing but had some very dodgy night bus experiences on my way home.
If you like anonymity and the 'bright lights' of the big city and you know exactly what you want to do and want a first in your subject then it might be for you.
If you prefer cosy pubs dominated by philososphy and english literature students having pretentious conversations putting the world to rights and a really active student union and a university community where people all know each other then it's not really for you in my experience.