I'm at Imperial now and I really, really enjoy it there. You're in central London and if you can't find something fun to do there then it's a problem with you rather than the city! London is expensive, but you're right next to three huge museums (Natural History, Science and V&A) and Hyde Park is only a few minutes away all of which are completely free. If you're more interested in clubbing and parties then London won't disappoint you!
The social side of Imperial is what you make of it. Some people don't like it and claim all the international students form cliques and ignore everyone else - this is completely false from what I have seen. The international students tend to make friends with people from their own countries because it makes them feel more comfortable in a new country, but they are all very nice and friendly when you talk to them. I've had no problem making friends and I've never felt excluded.
Imperial has a bad reputation for the social side. Because of its hardworking reputation a lot of people applying tend to be the quiet students who are often a bit shy in social situations. This means some of the students aren't as outgoing and can end up feeling lonely. As long as you make an effort you'll find friends really easily - I have friends now who like to go out partying every weekend and I have friends who like to stay home and have Game of Thrones marathons. There's a great mix of people at Imperial and as long as you put yourself out there you will find people to be friends with.
On my course I had three afternoons off a week in the first year so I got all of my work done then and I never had to work in the evenings (apart from around exam time!), which meant I had every evening free to go out with friends or just relax. My second year was similar and it looks like my third year will be the same as well (although I'm still waiting from a few of my timetables
).
My course (Biology) is known for being one of the more relaxed courses with some of the more outgoing students so my option may be different to other peoples. However, there's no reason you can't have the same experience on another course - you don't have to become best friends with people on your course after all. Some of my closest friends are doing different degrees.
My advice to you would be to take everything you hear with a piece of salt because people tend to remember bad things they hear. If you want to apply why not come along to an open day (or if you live close enough just come down for the day to look around)? You can talk to some current students and ask how they find the course, if you don't like the feel of it then you know it's not the right place for you