I studied there this year, whilst I was the youngest at 20 (I stress that was on a foundation degree for housing with people wanting to change careers) - this year is going to be different because I think the majority of full time undergrads are going to be under 21. I'm 21 in 8 days and I'm doing History part time (so I may see you there, RA23!). So I can't comment on the history - just changed from Housing to History, I've been told it's really fantastic though. I got my place through interview and my interviewer said the social situation is definately going to be different because they opened up part time (and added more full time) to those leaving sixth/college.
I can tell you that we have a lovely library and we can also use the other UoL libraries like UCL, LSE ect through SCONUL as well as others. Senate house depends on your department, but you can pay 80 per year to get access. We also get ULU access too. I think F/T stuff still might be taught in the evening, I'm not sure but I think it's 3/4 evenings a week for F/T. There's the George Birkbeck bar, which I haven't used personally so I can't say much. And basically last year my classes were in LSE's buildings near Aldwych - which was pretty cool.
But yeah, the usual stuff like societies and what not exist but I honestly couldn't tell you about how full time will work. I hope I've been some help though!