It really depends on what you want to do in the future. If you want to zone in on Actuarial Science, go to Cass. It offers you the most exemptions from the Professional Actuarial Exams. (8 out of 10 Core exams I think, don't quote me)
The bad thing about most London universities is that they can look be old and cruddy unlike those in Singapore (btw, hi-5. Saw your flag) and their accommodation fees are daylight robbery compared to other places in the UK. Its the compromise you have to deal with if you want the best degree for the course. The social life should not be that bad, after all you could go for the occasional outing with your pals beyond London if you are not studying. Another good thing about Cass is that the lecturers are the big players in the field so you are getting the latest updates. I went for the open day, the halls were absolute dank but the lecturers were open and friendly. Students there enjoy their uni life.
If you are geared on ActSci, Soto might not be the best as it is not on the accredited list although you have great peripheral activities.
It's really up to you though. Personally I would endure 3 years in a London uni with supposedly crap student life because I'll be at a better position with a Cass degree. Since enjoying uni years is 3 years but hunting for jobs is for life. Employers in the AciSci field do note which unis churn out better quality undergraduates and they recognize Cass as a prime source for good trainees. After all, Cass is one of the top business schools in Europe, not so much can go wrong with that.
Good luck in choosing anyway. :-)