I faced EXACTLY the same decision as you, and went for Durham. London is awesome, and I'd love to live there someday, but I figured that I can get the "London experience" later in life whereas the student experience is really limited to to uni, so I wanted to go somewhere more studenty. The collegiate system at Durham makes you feel really involved. I visited UCL and liked it but the friend I visited said she felt really segregated from other students, not because she was trying to put me off but she was sort of complaining about a club she'd joined. My dad went to Imperial and told me I wouldn't have enough money to properly "enjoy" London (if that makes sense). I'd also been to a Durham open day and had a great time. I did really consider UCL but I think I did the right thing in choosing Durham.
Please also see "Durham vs Edinburgh" in this forum, I defended Durham's nightlife there (college bars are awesome and there's one in the castle!). What Reblet says about the social scene is correct - there are so many clubs and socs to choose from, because there are university level ones and collegiate ones. If you like a sport or are interested in one, you can probably play for a team even if you're not amazing; I was NEVER sporty at school but I play for Hild Bede squash team in Durham and this year have been their captain!
By the way, you probably could live in in your second year without hassle but it's REALLY easy to find housing in Durham - that is, prices don't vary too much and there are loads of student housing areas, plus which there are more houses than there are students looking for them so we're desirable consumers!