That is understandable, it probably won't benefit them to actually keep that kind of data but it is always useful for applicants. I just find that sort of thing interesting to know!
I agree, I don't want to overgeneralise but I assume the reason that many LSE offerholders don't firm is because many (again, speaking in VERY general terms based on anecdotal experience, i.e. me stalking previous threads!) go on to Oxbridge/many debate UCL v LSE.
Did UCL not reply to that FOI request?