One advantage of Birkbeck is that where there are sufficient students on the full time course, they may run a module during the day and evening. This means you can attend the same class twice (time and schedule permitting). This is useful if you did quite get it first time round. Lectures do not start before 10 and lecturers are often around after 5pm.
If you are missing your student life fix, go next door to ULU (Birkbeck students get ULU membership), there are always events on there. You can socialise with UCL/SOAS students etc. I find it somewhat ironic that the under 21 years are worried about being surronded by "older people" and vice versa. When in education at university level, generally age is nothing but a number. The things you have in common, you have selected the same course and may have similiar long term goals. The differences such as educational background, life experience are always going to be there, but that means both parties have something to learn from each other.
Also because a lot of people at Birkbeck are working, it presents a good networking opportunity, for those thinking long term.
An American friend, who relocated to the UK was surprised to find that Birkbeck was a part time university as their perception was it have has a good reputation for research internationally.